The MacBook is very much Apple's entry-level laptop. It lacks the power of the MacBook Pro, but it is smaller - and is the cheaper option. You get pretty much everything you would from the Pro but without the lightning fast speed. The performance of the MacBook is great for web browsing, social media and emails, using Word, photo editing and DVD viewing.
The MacBook is faster than the MacBook Air but is much heavier. If you want to create music or movies, the improved performance of the Pro is probably the better option for a used Apple laptop. The MacBook lacks a FireWire port but you can connect most peripherals through USB. It does also have a larger hard drive than the MacBook Air.
The Macbook Pro has more processor power than most people would ever need and is perfect for music and video creation and photo editing. It has a FireWire, more gigabytes of RAM and the added benefit of an SD card slot. It comes in 3 screen sizes: 13", 15" and 17".
The Macbook Air is the lightest and smallest Apple laptop. They are extremely thin, weighing in at less than 1.5kg, but still provides a high-res screen and full-size keyboard. Macbook Air doesn't have the fastest Intel processor, but the flash storage speed and its graphics processor make up for this. For typical laptop use: browsing, email and word processing, the Air is perfect. There isn't a built in DVD drive, but you can connect an external drive using USB.