Most NHL teams go with 4 lines of forwards. Some teams break up the fourth line in favour of a 7th d-man.
Top two lines are generally your best 6 forwards in some form or another. The fourth line is either considered your checking line or energy line (depends on the coach). Usually the fourth line is full of your defensive minded forwards. Their role is to change the pace of the game. Third line really depends on the team. If you remember the Wings of the late 90s their third line likely could've been a second line on most other NHL teams.