The difference between masking and clipping is that clipping is 'spatial' and masking is 'logical'. When you clip you define the boundaries of a 3D space and all the members that are physically contained within that space remain active. The clipping bars you see on screen define the boundaries of the clipping box. You can move these clip bars with the mouse or use the 'clip to selection' or 'clip to frame' commands to automatically position the clipping box around the area of interest.
 
Masking allows you to select members to include or exclude from the active list. Mask To Selection makes all the selected members active while Mask Out Selection makes all of the selected inactive. The selection can contain any number of members in any part of the frame.
 
If you mask and clip at the same time, the active member list will comprise those members which will be visible due to the effects of both clipping and masking.