speaking from my own experience - which may or may not relate in any way to others experiences or the reality of a mudgather:

it can be as simple as two 'catters meeting and chatting

to a "meet and greet" type thing for multiples

to a really great music round robin

to a three day event with lotteries, music, meals, tours, concerts, all night sessions, gift exchanges and a heck of a good time.