Autofocus and manual focus basically do the same thing. They both adjust the focus of the camera lens... but, with autofocus, the camera determines the sharpest focus using sensors devoted to measuring it, it's faster than manual focus and it can vary depending on the camera.
In manual mode though, the photographer must adjust the lens focus by hand, it allows for greater precision when focusing, and also gives greater control over the shot's focus.
I think autofocus got a lot better in the last year than it was 10 years ago. It's been a while since I bought a real camera, but even with my older Sony camera I got so used to autofocus it's quite and make using the camera much easier. I can imagine there are plenty of people who prefer manual focus to get the perfect picture.