Focus is the ability to sort different visual and auditory stimuli so as to prioritize actions. It often requires ignoring certain irrelevant information in order to successfully allocate attention to another. This ability is developed through the years and is action-specific.
Example: One of the most interesting manifestations of this ability is "The Cocktail Party effect": have you ever stood in a room full of people chatting and suddenly noticed someone discussing your name. This ability to sort out certain information from other irrelevant ones is the ability to focus.