Slugs - Help me to understand

[eluser]Phil Sturgeon[/eluser]
A slug is simply a unique identifier that means more to a user than 459372 does. It also protects people from mining your data by incrementing a number in a URL and saving all your stuff, or being able to work out how many sales you are making a'la the ridiculous way Magento handle their sales.

You should be making your slugs unique, by either saying "this title is already taken" or by adding -2 to the end of the slug if you've found a match in the db.

Also make the field into an index key or your database will get SLOW.