Bottom line is, if you want to listen to others (and perhaps do a toon or a song) then go to a folk club. If you want to take part, following others who are singing or playing, and doing your own songs/toons, go to a session. The latter always for me, I don't do spectator very well, I want to be plunking and hollering along with the others.
Above all, a good session (such as the Six Bells in Barrow, North Lincs) welcomes all abilities. It is really hard when starting to play an instrument or sing to get practice with others. Listening to music on a CD etc is OK, but there is no substitute for a session with live musicians, many of whom are happy to pass on tips & help newbies. To my mind a session is where the Folk Tradition is kept alive and handed on in its most practical and enjoyable form, with people listening to and learning from others playing and singing live.