For a really brief explanation: Regular collections store objects. The system doesn't know what kind of object is stored, so you have to cast them to the desired type when you work with them. Generic collections declare what kind of object is being put in at the time you create it. Then you always know what is there. it's like the difference between an object array and a String array.
I would definitely check out the list of links on the page PK posted for a more thorough understanding.
