I'll let the pros either elaborate or correct me, but here's my $0.02:
circular and triangular are similar in that they both focus on using angles to approach or evade the opponent. Circular footwork tends to have more stance switching, i.e. starting in orthodox, an opponent lunges forward towards your center line, so you pivot on your back leg and step backwards putting yourself in southpaw. Triangular footwork would have you move backwards and to the right, so you'd be in a similar position in relation to the opponent but in your orthodox stance. Both have advantages and disadvantages depending on the situation. FWIW, there's a lot of bullshido involving circular footwork, so use discretion.
Linear footwork is more related to the mechanics to move in a direction, like which foot to move first and how to transfer weight so your movement is efficient and you aren't off balance. It can also be related to grappling or fencing since both sports require an efficient penetrating step to cover forward distance properly.
Boxing footwork is all of the above as well as the mechanics of moving your feet as you strike so the power is transferred from your feet to your fist.
