Pictures, videos, downloads, music. You can also move some apps (or parts of apps) to the card. I use an app called AppManager III (used to be called App2SD. It's in the Google Play store). It will analyze all your apps and tell you which ones can and can't be moved to the card. Also, when you install a new app or upgrade an app, you will get a notification if the app is able to be moved to the card.
You can also backup apps to the card using Titanium Backup. You could probably find an app out there to backup your contacts, texts, etc. I usually back that stuff up to the cloud.