B-cells are white blood cells that begin their lives in the bone marrow. These immune system cells are immature when they leave the bone marrow. They don't develop into fully-functional B-cells until they reach areas of the lymphatic system, including the spleen and the lymph nodes. Once developed, B-cells circulate through the blood and perform a sort of "immune surveillance," keeping an eye out for things that should not be in the body.
The two most common types of B-cells are plasma cells and memory cells. Plasma cells are responsible for producing antibodies. Antibodies are proteins that can latch onto foreign invaders and tag them so that other immune cells will recognize them as invaders.
Memory B-cells have a prolonged life span and can thereby remember specific intruders they've seen before. This is where the name 'memory' comes from. The second (or third, or fourth, or more) time a particular virus, bacteria or other foreign invader enters the body, the memory B-cells will help the immune system react much more quickly. If the immune system is able to react quickly, the invader will not have time to multiply and will be destroyed before noticeable symptoms develop.
B-cells are a part of the immune system known as acquired or adaptive immunity. This means that to become fully functional, these cells must "learn" how to do their jobs.
