Focus -- the adjustment of the lens so that the image you are video taping is sharp and clear in your view finder or on your monitor.
Auto Focus -- a mechanism built into all modern video cameras which enables the camera to refocus constantly, keeping everything in your frame in focus.
Unfortunately, auto focus seeks the brightest object in the frame to focus on, or the object closest to you. So if you're video taping a singer, and the lights are reflecting off the cymbals in the background, the auto focus may continually throw the singer out of focus as it tries to focus on the bright reflection.
Sudden movement can effect the auto focus mechanism similarly.
Manual Focus -- Your camera will have a switch which allows you so select "Auto" or "Manual." If you choose to focus manually, you will have to make use of the focusing ring on the lense to keep the camera in focus. This takes a little practice, but will give you better video in many instances.
Focus Tip -- Lets assume you are at the back of an auditorium, video taping a singer and some performers. With your camera on a tripod, select "Manual" focus. Now zoom in as tightly as possible on the stage. Select an object on the stage and focus on it carefully. You should now be able to zoom in and out freely, and the camera will remain in focus throughout the zoom. If you change the position of the tripod, you will have to repeat this procedure.