From BlenderWiki
Vertex Groups panel
This panel is available in edit mode, when the object to be edited is a mesh, and there is an active vertex that belongs to a vertex group.
You can see a series of number buttons that show (and let you change) the weight of the vertex in all the vertex groups where it belongs.
Next to each one of these buttons there is a C button. Clicking on one of these, the weight of the active vertex in the corresponding group is copied to the rest of selected vertices. However, vertices that don't belong to that particular vertex group previously remain unchanged.
Normalize applies a correcting factor to all weight values of the active vertex, so that they are normalized, that is, adding all those values together gives a result of 1.0.
Copy copies all the weights of the active vertex onto the rest of selected vertices, so that these lose their previous weights and vertex groups information.
