A bevel is any flat or curved surface ground into the blade to reduce its thickness. Knives normally have two that matter: the primary bevel that thins the whole blade, and the secondary bevel that forms the actual cutting edge.
The primary bevel is the big one, running from somewhere near the spine down toward the edge. It is what people mean when they say a knife is flat ground or hollow ground.
The secondary bevel is the narrow one right at the apex. It is what you actually sharpen. On most factory knives it is a millimeter or less wide.
A bevel is described by its angle, either per side or inclusive of both. For the angle each maker actually states, see what angle knife makers sharpen at. There is no free lunch: every degree of sharpness you gain is a degree of durability you spend.
A micro-bevel is a way of having both, a thin primary geometry with a slightly steeper final edge for durability.