(2-part post):
There are no fixed rules but here's a (very) rough guide:
B&B establishments are usually private houses where the owner lets out one or more rooms, at a daily rate. To qualify as a 'B&B' they don't need to provide anything other than exactly what the name implies, i.e. 'bed' and 'breakfast'. It's unlikely that you'll find a bar or any public rooms. Some B&Bs might offer additional facilities, such as evening meals, but such places would probably consider dropping the B&B title, in favour of one of the others. (A few B&Bs still adhere to rules which require guests to be away from the premises during the day but this practice, which used to be common in British seaside resorts, is becoming increasingly rare).
Guest houses will often (but not always) offer additional facilities to those offered by a B&B. You might find a public lounge, possibly with a licensed bar. Some (but not all) will offer 'bed , breakfast and evening meal', while others might provide 3 meals a day, under the title 'full board'. The meals provided in such establishments might simply be the dish of the day, rather than providing the choice you'd get in a restaurant