Essentially there are two types of web sites, Static and Dynamic. There are pluses and minuses to both, what you choose simply depends on what you are looking to do, how much time you have to devote to your site and your skill level.

A static site is one that is coded in plain HTML (or Flash for you crazy people out there) and requires a general knowledge of HTML (or at very least a good WYSIWYG editor) for maintenance. The majority of sites out there fall into this category.  Some of these are professionally designed and maintained by the design company, some are professionally designed and maintained by the site owner and some are completely designed and maintained by the site owner.Unfortunately with most of the sites that fall into this third category,  the site users can probably tell because the site does not offer the best experience. This is not always the case, in fact I have run across a number of  sites developed this way that are just fine but not everyone has the time to work out the many design issues that plague some sites.

The primary advantage to this type of site can be cost and accessibility. The obvious disadvantage to this type of site is that the site owner needs to have a decent knowledge of HTML (even if they use a WYSIWYG editor) to ensure that you are able to avoid some issues either that or the site owner needs to employ someone with the appropriate knowledge (as a contractor or FTE) to get the job done.

A Dynamic site is one that has the content served up from a database as each page is requested. Most online stores and your more interactive sites out there fall into this category. Dynamic sites are usually programed in some form of ASP or PHP (although there is cold fusion, JSP and other XML based systems in regular use as well). These sites use some kind of database, like MySQL and MS SQL (even MS Access is used for some smaller sites), to house the site information.

Like with static sites, there are advantages and disadvantages to Dynamic sites as well. The biggest advantages are scalability and easy ongoing content management without the need for a third party. The biggest disadvantages can be the up-front cost can be somewhat higher than that of a static site and historically there have been more SEO issues with some dynamic sites. The cost disadvantage is mitigated by the fact that dynamic sites offer a return on that investment over time by not having the need to pay someone on an ongoing basis to make content updates and changes. In general, dynamic sites can be especially advantages for online stores, newsletters and sites with content that changes frequently.

A good balance between these two can be found with WordPress. Although it technically falls into the  dynamic category it can come without the high cost of development (but keep in mind that a good custom theme can get pricey in the same way any site design can depending on what you want) and with options to help expand overall functionality and minimize if not eliminate any SEO related issues that dynamic sites have had in the past.

The bottom line is that you need to think your site all the way through from the beginning. No matter what kind of business you are in, you just need to be sure to fully consider exactly what you want out of your site so you can determine the best way to go before you start the process.