HTTP/Web Server
The term “Web server” can refer to either hardware or software. Almost any computer can be used as Web server hardware, and Web server software choices are countless. According to Netcraft's Web Server Survey, the most widely used Web server software on the Internet comes from Apache, nginx, Microsoft, and Google respectively.
Hardware
Any machine hardware where a website is stored. It is usually fast processing and has computing resources like a lots of storage space (or set up may be a separate attached storage that could be local or virtual or networked). The webserver has a permanent IP (Internet Protocol) address that uniquely identify this computer on the Internet. Generally, it is easier and more cost effective to hire a Web-host company to store Web sites if inhouse data center is not maintained, however this decision is solely based on business requirements and availability of resources like inhouse IT-staff- administration of servers more specifically. Another emerging trend is to outsource all or some of IT servers, workloads, administration and hosting needs to cloud service providers who are specialized in off-the-self and customized services-offerings based on customers' requirements. The customers (businesses) pay for used resources with pre-agreed SLA's (service level agreements) that ensure availability of service, data security and integrity.
Software
Web server software is a program that runs on the hardware that stores Web sites. The server software uses the Hypertext Transfer Protocol to send the Web page to the Web browser, where it is displayed to the end user. Web servers typically run two types on operating systems- Linux, and Microsoft Windows. Most web servers today operate Linux, and most websites are hosted on Linux Servers. Servers with Linux and Windows installed on them also run other key software programs for called Daemons that run in the background (e.g. HTTP Server, FTP Server, Email Server, Database Server). Servers may also run other programs installed on them like content management systems (CMS) like WORDPRESS, Joomla and Drupal, these software programs are usually offered free of cost with web hosting plans.
Options
Leading Web servers include Apache (free, open-source and most widely installed Web server), Microsoft's Internet Information Server (IIS- Microsoft proprietary and licensed by Microsoft) and nginx (pronounced engine X- is a free, open-source) from NGNIX. Other Web servers include Novell's NetWare server, Google Web Server (GWS- proprietary web server software by Google) and IBM's family of Domino servers Web servers often come as part of a larger package of Internet and intranet-related programs for serving email, downloading requests for File Transfer Protocol (FTP) files, and building and publishing Web pages. Considerations in choosing a Web server include how well it works with the operating system and other servers, its ability to handle server-side programming, security characteristics, and the particular publishing, search engine and site building tools that come with it.
HTTP Server
A web server is "a computer system that processes requests via HTTP, the basic network protocol used to distribute information on the World Wide Web. The term can refer to the entire system, or specifically to the software that accepts and supervises the HTTP requests." (Wikipedia)
Apache HTTP Server
The Apache License meets both the Open Source Initiative's (OSI) Open Source Definition, and the Free Software Foundation's definition of "free software". Apache projects will never charge a fee for downloading or using their software (https://www.apache.org). While Apache httpd is most often seen running on Linux, it can also be deployed on OS X and Windows. Apache is, unsurprisingly, licensed under the Apache License version 2. The web server itself uses a modular architecture, in which extra modules can be loaded to extend its features. For example, loading the mod_proxy will allow for a proxy/gateway on your server, and mod_proxy_balancer will enable load balancing for all supported protocols. As of version 2.4, Apache also supports HTTP/2 through a new module, mod_http2 (Wikipedia). As the Apache HTTP Server has been the most popular web server since 1996, it "benefits from great documentation and integrated support from other software projects." Because Apache has been popular for so long, support for the server is fairly ubiquitous. There is a large library of first- and third-party documentation available for the core server and for task-based scenarios involving hooking Apache up with other software. Apache, in general, will have more support from third-party projects simply because of its market share and the length of time it has been available. Administrators are also somewhat more likely to have experience working with Apache not only due to its prevalence, but also because many people start off in shared-hosting scenarios which almost exclusively rely on Apache due to the .htaccess distributed management capabilities (apache.org)
NGINX HTTP Server
Pronounced “engine-x,” is the second most popular on the Internet. It's open source and can run on the major operating systems like UNIX, Linux, Windows, and Mac OS X. NGINX is a free, open-source, high-performance HTTP server and reverse proxy, as well as an IMAP/POP3 proxy server. The developers of nginx claim it is better than Apache regarding low memory usage when serving large numbers of site requests. Apparently, companies like Netflix, Hulu, and Wordpress.com agree; all use nginx server software (Netcraft Ltd.'s Web Server Survey). NGINX relies on an asynchronous event-driven architecture to help power its goal of handling massive concurrent sessions. It has become a very popular web server among administrators due to its light resource utilization and its ability to scale easily. Nginx is experiencing increased support as more users adopt it for its performance profile, but it still has some catching up to do in some key areas. In the past, it was difficult to find comprehensive English-language documentation regarding Nginx due to the fact that most of the early development and documentation were in Russian. As interest in the project grew, the documentation has been filled out and there are now plenty of administration resources on the Nginx site and through third parties. Regarding third-party applications, support and documentation is becoming more readily available, and package maintainers are beginning, in some cases, to give choices between auto-configuring for Apache and Nginx. (Netcraft Ltd.)
Apache Tomcat
Apache Tomcat is yet another flavor of an open source Java servlet container that functions as a web server. A Java servlet is a Java program that extends the capabilities of a server. Although servlets can respond to any types of requests, they most commonly implement applications hosted on Web servers. Such web servlets are the Java counterpart to other dynamic web content technologies such as PHP and ASP.NET. Tomcat's code base was donated by Sun Microsystems to the Apache Software Foundation in 1999 and became a top-level Apache project in 2005. It currently powers just under 1% of all websites. Apache Tomcat, released under the Apache License version 2, is typically used to run Java applications. It can, however, be extended with Coyote, to also perform the role of a normal web server that serves local files as HTTP documents. (Wiki)
Microsoft Internet Information Services Web Server
Computers running Microsoft Windows can use the IIS Web server. IIS is required for Web sites that are programmed in ASP.NET. IIS is a proprietary server -- that is, not open source -- and you need to purchase a license to use it. IIS is not installed by default but is available through "Add and Remove Programs." Like Apache, Microsoft released IIS to the public in mid-1995. Although Apache has the lead in the general market share, IIS is the preferred Web platform for Fortune 1000 companies.
Google Web Server
While GWS is amongst top six most used server software on the Web, according to Netcraft's August 2012 Web Server Survey, it is not a server the public can use. The reason it has such a huge market share is due to Google's user-generated content sites like Blogger.com, Google+, and Flickr.
Node.js
Node.js is a server-side JavaScript environment for network applications such as web servers. With a smaller market position, Node.js powers 0.2% of all websites. Node.js was originally written in 2009 by Ryan Dahl. The Node.js project, governed by the Node.js Foundation, is facilitated by the Linux Foundation's Collaborative Projects program. The difference between Node.js and other popular web servers is that it is primarily a cross-platform runtime environment to build network applications with. Node.js applies an event-driven architecture capable of asynchronous I/O. These design choices optimize throughput and scalability in web applications allowing to run real-time communication and browser games. Node.js also highlights the difference in web development stacks, where Node.js is clearly part of the HTML, CSS, and JavaScript stack, as opposed to Apache or NGINX which are a part of many different software stacks. Web Server Security "Both IIS and Apache, if installed as directed by the developers, are relatively secure," states Roger A. Grimes, InfoWorld's Security Adviser. "Most malicious Web site infections are the result of administrative mistakes and buggy applications --- not the underlying Web server software." In a study conducted by Google, malicious software (malware) was evenly distributed by both servers.
(Ref: Wikipedia, Stackoverflow and Google)