The A refers to the bulb shape. A is for arbitrary and is the shape that we all think of when we envision a light bulb (C is for candle, G is for Globe, etc.) The number 19 designates the size.

Buying a light bulb can be complicated. If you don't take the time to figure out what type of bulb you need, it may not match the color of the other bulbs in the room or even work in your fixture.