Hi Melissa!
Downtown Disney (DTD) is open to anyone who would like to visit and have a look around. There is no admission charge to get into DTD, and parking is also free! It is a lot of fun to browse all of the shops, and there are many restaurants to choose from for a great meal.
Be sure to check out the World of Disney store. It is the largest Disney character store in the world, and has rooms full of great merchandise. It is also home to the DTD location of the Bibbidi Bobbidi Boutique.
While there is no admission charge to enjoy the atmosphere and shops at DTD, there are a few venues that charge an entrance fee. To enjoy the interactive environment at DisneyQuest, you will either need to purchase separate admission, or have the "Water Parks Fun & More" option on your park tickets. If you have the option added to your tickets, it includes admission to DisneyQuest. Cirque du Soleil requires a ticket purchase, as does the AMC movie theatre.
Have a great time at Downtown Disney!