BlackBerry Appworld is the equivalent of the iTunes Appstore or Android Market place.
You can search for apps, try before you buy or get free apps here.
You can see screen shots and read reviews to help you make a decision. Appworld can be downloaded to your device from here: http://www.blackberry.com/appworld
Appworld will keep a record of your downloads and purchases via your BlackBerry ID so if you change devices or wipe your device you can download the apps again without having to repurchase them.