This diagram shows what the parts of a bicycle are called. Knowing the bike component names (nomenclature) and how to correctly identify them will help you when you need to explain something to a mechanic working on your bike, shopping for upgrades and when you're talking about bikes to other cyclists. Another excellent resource is Sheldon Brown’s bicycle glossary.
Notes: Hidden by the crankset in this picture is the bottom bracket. There are actually two bottom brackets.
You call the bearing assembly that the crankset spins on, the bottom bracket (marked A in the photo; the mechanic is holding the end of the bottom bracket axle that runs through the frame and that the crankarms attach to). Mechanics and cyclists often say “BB” instead of bottom bracket.
And you also call the part of the frame that the bottom bracket bearing assembly is screwed or pressed into, the bottom bracket (marked B).
Incidentally, often the serial number of the bicycle is stamped into the bottom of the frame bottom bracket. Other common bicycle serial number locations include the left rear dropout and on the bottom of the frame seat tube above the bottom bracket. It’s important to know your serial number for registering your bicycle to prevent it from getting stolen.