Well, gatherings are just when a bunch of people meet up in one place and discuss something.
Clans, are typically groups of cats, like in the 'warrior cats' if you have heard of those books.
A medicine cat is the cat of a clan, whom heals/take care of the wounded members of the group.
Feral Heart is basically a all RPing game where you get to be a wolf or a canine and just role play with other users. Some users set up group to make there Roleplaying more interesting and some groups are based off of books such as Warriors.
You can also look at the Feral Heart manual to see if that could also give you a better understanding
FH Manual (http://feral-heart.com/index.php?option=com_content&view=article&id=4&Itemid=33)