Add-ons are miscellaneous items that can be bought from the Pledge Store or traded by other players while Star Citizen is in alpha. These items can range from ship upgrades to digital downloads like soundtracks. You will sometimes see add-ons packaged with Standalone ship or Game package.
These add-ons are used to upgrade an owned ship with different options, such as, the Retaliator that has exchangeable compartments or change the cosmetic appearance.
Hangars are player owned locations that you can use to store ships, show off cosmetic items, and explore your ships. These can be upgraded from the base SelfLand hangar. Sometimes specific hangars are attached with some ship packages.
The pledge store also offers some add-ons that are related the the game but aren't game items themselves. These would include game soundtracks, novellas, and manuals.