a document type definition(DTD for short) tells the browser what type of web page your sending. you have 3 types for each version(transitional, strict and frameset).
transitional is great for backwards compatability and such.
strict is the best, it offers full compliance to the standards if it passes.
frameset is the same as transitional, but frameset, frame and iframe elements are permited.
your proberly better to google it though, since I'm terrible at explaining things.