TBS Missions are one of two types of TBS, the other is Skirmish.
TBS Missions are beautifully designed TBS combat dungeons with specific objectives you need to accomplish in a given amount of time.
You can read more about TBS combat on the main TBS page. This page is a list of TBS Missions.
There are a number of good guides for TBS missions on the web and you can even find video walk throughs for most on YouTube. My (Smashix) favorite set of TBS guides are at Secrets of Atlantica, a very good MMO blog with lost of good info about Atlantica Online: