Overlap is the portion of the engines rotation for which the exhaust valve is still open right at the end of the exhaust stroke, and the intake valve is also open to give a head start to the incoming charge. The exhaust gas is whistling out of the exhaust port at the same time the intake charge is starting to trickle past the intake valve. Lots of overlap works great at high rpm because more intake charge manages to cram itself into the cylinder, but lots of overlap will also make the engine run badly at low rpm, as exhaust gas manages to make its way back up the intake manifold, diluting the incoming air/fuel charge, and depositing soot on the intake runners, carburetor, etc. If you have more than about 40-50 degrees of overlap, you're looking at something more like a race cam than a street cam.
Lobe centre angle, on the other hand, is the number of degrees between the point where the intake valve is at maximum lift and the point where the exhaust valve is at maximum lift.
The two angles are related - for a given lobe design, as the LCA gets smaller, the overlap gets larger. Smaller degree LCA's usually give you lots of low rpm torque, but not as much high-rpm power. Larger-degree LCA's give you lots of high rpm power, but poor low-rpm torque.