What's the difference between flip-flop and latch?

Keep this in mind, what is the main distinction between a latch & a flip-flop?

A flip-flop and a latch are different in that they can be level-triggered (outputs change when inputs change) while Flip-Flop is edge-triggered (state changes only when control signals go from high to low to high or low to low). Your design should always pay attention to latches.

You may also wonder if flip flop or latch is better. A flip flop is a latch that has an additional control input. This additional control input controls when the circuit's state is to change. This unit can either be an enable input or a clock input. Flip flops are preferred to latches.

Another question is why flip-flop is called latch?

If an input is used for flipping one gate (making it go high), then the other gate will flip (go low). Hence, "flip flop". If the clock input is in the right state to enable the first latch it will track that state, while the second D latch will keep whatever state it's currently holding.

What is the difference between register and flip flop?

The flip flop should be constructed with logic gates so that it can hold and not change, unless externally changed to a 0 (or any 5 or 12 or 3 voltage depending on the system). Register is a flip-flop in its core.