When you're choosing your Web server software, you need to be aware of many elements of your site. Things such as the operating system you want to run, the number of page views you expect, and the level of support and security you require will all influence your decision. And of course, cost will always come into play.
What operating system your server will run on will influence what server you buy. There are some servers that only run on specific OS's. The most popular OS's for Web servers are: Unix, Windows 2000, Linux, and Macintosh. I recommend small businesses stick with Linux, unless they can afford Unix. Windows 2000 has a lot of security flaws, and Macintosh servers are not very common so you won't have a lot of support from the Web community.
Security should be your number one concern when considering a Web server. Your Web site is very public, and being hacked is a public way to reduce your customer confidence. Once you have a secure Web server, you also need to focus on how you configure it. Many Web servers come with defaults that are not as secure as they could be, and that could make your site vulnerable. Finally, when thinking of security, you must stay up-to-date on patches and security issues.
Scalability is the ability of your Web server software to grow as your Web site grows. Having a slow server can cost you both existing customers and new ones, and you often only have a few seconds (less than 10) to grab and hold your customers to your site. You do not want a Web server that will bog down after 1,000 hits.
SupportOne of the most popular Web servers in the world is also completely free, but with that comes a price. That price is a lack of formal support. If you do not have a large IT department, already staffed to support the server you're going to choose, then you should probably look into either a server with support included, or a third-party support contract for a less well-supported server.
CostCost is usually the first thing that people look at when trying to decide to buy something. With Web servers, it should be the final thing you look at. There are servers available that are free (or free with an expensive operating system) and there are Web servers that are expensive software bundles. Limiting yourself to a specific Web server because of cost could end up hurting your site because of a lack of scalability or functionality, or not be maintainable because of lack of support.