Back button focusing, what's it for, how to use it, and what are its advantages
With back-button focus, you use a button on the back of the camera to focus, and use the shutter button just to set the exposure and take the picture. The shutter button never tries to achieve focus because it is re-configured not to focus.
I made the permanent switch to focusing using the Back button a while ago, and I’ve become a huge advocate for this type of focusing. Alongside using the Manual exposure mode, I'd say it was one of the most revolutionary changes I ever made to how I take pictures. It seems like such a small thing, yet it’s such a huge difference in the way your camera works, and should probably be the standard setting on all DSLR.
There are several advantages to this technique, and they are explained in this video, however it must be said that some people try back button focusing, and just don't get on with it, so it's not for everybody. But it is worth investigating, you could be missing out and might just find that it's something of a revelation!