Focus is the point where the rock starts to fracture. It is referred to as the origin of the the earthquake. Also, known as the hypocenter, vibrating energy from the hypocenter travels in all directions. It's the point where rupture starts.
Epicenter is usually misinterpreted as the focus, but epicenter is actually the point on land that is directly above the focus. It isn't the starting point, but it's the point on the land that's above the starting point.
