Armed with an idea of the type of site you'll build, and who you are building it for, it's time to gather content.  Here's where we introduce a new term: Assets.  Assets are the pieces that, when combined, form the content on your site.  They are the text, calendar items, images, audio, video . . . basically the pieces that create the whole.

You may not have all the content ready, but it's a good idea to have a sample of each type.  This will give you an idea of how to present what you have rather than create blank spaces and attempt to wedge things into them.  It may also give you an idea of how much content is available.  This effects layout.  You may believe you only have a few events to promote over the year, so a small space is set aside on the home page to display these few events.  But you discover that other groups also have events to promote.  That short list needs more space.  And this can effect layout.  And now that there are other groups events, who will be managing the content for those events? 

Having as much of the content and assets available also helps make sure the site contains the right technology.  A real estate site may require a special video player for virtual tours.  There may be limitation on the size of the files that can be uploaded, or the size of the player itself.  These are the things that need to be addressed early in the design process - not in the development phase.