Each musician has a great deal of freedom in the way he interprets a tune. The mood, the other musicians around, the general ambience of a room, whether there's a pint waiting on the table, or even the weather might change how a musician plays a tune. This is why it's funny to hear people try and play the music after learning it only out of a book - they play it the same way every time, and they don't play with that distinct, strong rhythmic feel. Traditionally, the music is learnt by ear, without the aid of written music. Though favorite tunes might differ from region to region, there is a broad base of 'common' tunes.
Sessions are mainly, then, for the players.
Even when they occur in public settings, they don't look like formal performances. You're likely to see folks in a session sitting around in a circle, playing with and talking with each other. There's no expectation of applause. It's rare to hear just one, strong, fine tune by itself - but it can happen. You might not put loads of tunes together if you're, say, playing tunes with 5 parts to them.