As the name suggests, a lobster clasp resembles a lobster claw. A spring-loaded arm stays shut until the user pulls on a lever to open it. These are often easier to use than a spring ring (see below) because they are more elongated, while the spring ring is round.
A spring ring opens by pulling on a short arm, which slides a piece of metal along a circular ring, opening a gap, allowing it to be passed through a ring on the other side of the piece of jewelry. Letting go of the arm slides shut the piece of metal, covering the opening again. The spring keeps the clasp from opening accidentally.
Typically used for bracelets, a fold-over clasp is quite common. The clasp passes through a small ring attached to the opposite side of the piece, then the bottom piece folds over on a hinge and clicks into place, trapping the ring. Occasionally these have a push button for extra security, which must be pressed before it can be undone.
A bar attached to one side of a piece of jewelry fits through a single, large ring on the other side. If the bar is shorter than the ring's diameter it is a poor fit and not secure.
This type of clasp is very secure. It is comprised of two pieces, on one side is the "box" and the other is the "tongue". The tongue is "V" shaped, which allows it to hook around a pin in the box. Once the hook is on the pin, it can be pushed directly into the box, where it then clips onto a second pin. If the clip should fail once the tongue is inserted into the box, the hook will still catch on the pin, keeping it from falling off.