On the php|architect blog there's a recent post from Jayesh Wadhwani asking a question developers all over the world wonder every day - to use a framework or not to (and what's the benefit)?
A framework is usually thought of or defined as an underlying structure. You could imagine a wooden structure, sort of a skeleton when a house is being built. This provides a guide, structure and flow to build the house. A programming framework pretty much does the same thing. A programming framework provides for a structured and disciplined programming which results in a more consistent output from a programming team.
He talks more about some of the things that come with framework use like the utility and "housekeeping" code as well as code you know has been tested and used by other projects successfully (especially with something like the Zend Framework. Remember, though, there's bad that comes with them - a possible steep learning curve and overhead that could be caused by using the tools it gives you badly.