Transferring trains on one journey, how many tickets ?

eg. I want to travel from A to C, but am required to change trains at B. How many tickets do I need to buy ?

Do I just buy one ticket at my starting point (a "transfer ticket" ?) similar to being on a plane that has airport transfers ? ...... or, do I need to buy two tickets at the starting point ie. one from A to B, and also the one that is for the next train, B to C ? ...... or, do I buy my first ticket for A to B at point A - then - get off the train at B and buy my next ticket there to get me to C ?

It's a bit confusing.

Thanks.