Downtown Disney is a collection of shops, restaurants and entertainment. Some of the places there are:
World of Disney
ESPN Zone
House of Blues
There are plenty of others. Go to Disney.com, Vacations, Disneyland Resort, and you can find more info, including a list of all the places there.
There is no cost to get in. WDW in Orlando also has a Downtown Disney, and they have an area called Pleasure Island, which I believe does have a cost, so that's probably where you heard that.