If you go to settings/applications/running apps, take note of the ones that are running. Go to settings/about phone/Battery use.
If you just want to see which apps are running without having to go thru all that, download OS Monitor. It will show you what apps are using your system and what percentage. Network interface usage. It will give you Connection info on each app that uses your data connection. It will give you CPU, battery, and system info. last but not least it will give you the dmesg and the logcat. Lots of options in the menu.