[eluser]dmorin[/eluser]
n0xie's response is pretty good, but limits you to one category per post. Most blog engines allow multiple categories per post so if you're looking for that, your posts-to-categories relationship will be many-to-many instead of one-to-many so you'll need an intermediary joining table like he created for tags. His version of categories also allows for hierarchal categories which may or may not be more complex than you're looking for right now. I also don't like how in his example, he uses 0 as the parent id to represent that a category has no parent. This should instead be NULL so that parent_id could have a foreign key relationship with the cat_id column.
Do you understand table relationships and how to do joins or do you need further explanation on that?
