As I understand it, a tavern is a public house, hotel or any form of hostelry where drinking takes place
A snug is a smallish room - often reserved for regulars - within that public house, hotel or hostelry
Within said public house etc a number of activities might take place
1.a session - usually an informal get together of singers and/or musicians
2 a singaround - hosted by a peer or organiser, the singing/playing moves informally around the room with everyone invited to take part and open to everyone although there may be a small fee
3a singers night - again open to everyone to sing and play at the discretion of the organiser - these are sometimes accoustic but often open mike evenings
4 Guest night - an evening where a paid (usually) performer will entertain for the majority of the evening with the rest of the time filled by local singers and musicians (at the discretion of the organiser)
5 Concert - sometimes pay at the door and open to everyone - sometimes admission by ticket only - generally open to anyone who can afford to go