what are blocks and heads?

haha, as you can see, i'm not very familiar with the inside parts of the engine. i'm sure someone can help me cuz i see these two words thrown around alot!!!

lots of time, i hear ppl say "h22a head with so and so block" or "keep the block and just switch the head". something along those lines; what are blocks and heads? also, if they fit inside a big category, such as, well, i'm not sure, engine parts, something LIKE that, could you please inform me of the other parts under that category. i want a swap, but i want to know every single specific before i even start looking for an engine.

thanks alot for the help!!!