In versions of TweetDeck prior to v0.32 it was possible to create TweetDeck groups to organise your friends' updates.

With the introduction of Twitter Lists in v0.32, the creation of TweetDeck groups has been disabled, in favour of creating Lists instead.

Any Lists that you have previously created are still available for you to use, but no new groups can be created.

Therefore, if you are not sure what TweetDeck groups are all about, then read up on Twitter Lists instead!