A dynamic site is one that is written using a server-side scripting language such as PHP, ASP, JSP, or Coldfusion. In such a site the content is called in by the scripting language from other files or from a database depending on actions taken by the user.
Advantages : The main advantages of dynamic sites are that by connecting them to databases you can easily pull in information in an organised and structured way to create product pages or categories of related products sorted in a variety of different ways depending on how the user wants to view them.This ability to connect to a database means that you can also create a content management system - an interface which allows the client to input and manage data via a web-based series of administration pages. That content can be text for their pages and images to go along with the text, or items in their product range with categories, specifications, short and long descriptions, images, etc. In both these cases it can be as simple or as complex as the client requires.
Disadvantages : The design of a dynamic site is more fixed than a static one because many of the pages are essentially a template into which data and content is poured to create multiple pages of a similar type. So for instance all your product pages will be essentially the same page layout with different data being displayed. While some customisation cabability can be built in it is usually quite limited, such a selecting from a set of pre-defined options. Individual layout changes to particular pages are not usually possible.Costs are higher initially than for a static site, and additional functionality may also cost more, particularly if it's something that wasn't envisaged originally and requires re-writing of the core code or database.