Still confused about these "custom post types"? We already have categories for Posts and we can use various techniques like post_class to target certain posts with CSS and do different styling. We can also use conditionals to treat posts in different categories completely differently. Why is this going to help?
The big difference is that it actually adds sidebar links to the Admin side specifically for these new types. And beyond that, you can actually declare custom data types for different types. Instead of just "Add Post" and "Add Page", you can have "Add Real Estate Listing", which only will ask for "Property Name", "Description", "Price", and "Sale Status". Powerful stuff!