Jamming is a great way to learn to play publicly. For those of us 'classically trained' musicians it is a bit scary to play-by-ear without music but the fellow musicians are (almost by definition) the kindest audience you will ever have.
Everyone sits in a circle and each player in the circle will get a turn to choose a piece to play. This player is then the song leader. Bluegrass jams allows for individuals to deviate from what others are playing (i.e. experimentation).
The intent is to have everyone play along, either melody, or harmony (if you know it), but sometimes even if you haven't memorized the music to a piece, you can harmonize by simply playing notes that are in the chord of each measure (chord grazing).
Sometimes you just "Chuck" (keep your fingers on all the strings to dampen them and just drop the bow onto the dampened string to give a "Chuck" sound. You do this on the up beat (second and fourth beat of 4/4 time), so that you give the "Chuck" to the "Umm-Chuck Umm-Chuck" of the song.
Fiddle tunes typically have an A part and a B part. It is extremely common for everyone to repeat first part before moving onto the second part, referred to as AABB.
If the song leader gives the nod to another player, that other player may take the lead and do a 'break' or kind of solo - which is typically a variation on the original theme.
When the song leader feels the tune has been played enough, they will raise their leg to signal to the other players that the tune is coming to an end.
In Irish jam sessions, everyone plays together in unison. (i.e. there is no harmonizing or chucking as in Bluegrass jams)
Typically, there are three pieces which are linked together in a set Example: Everyone will play Swallow Tail Jig together, and on the signal of the jam leader everyone will transition directly from the end of Swallow Tail into the next piece (lets say Road to Listonvarnon) without missing a beat, this piece will be repeated until the leader gives the signal and everyone transitions to the third piece.
What's the difference between a violin and a fiddle?
The nut at the end of the bow.