Insert Vs. Overlay Editing

Both Insert and Overlay involve adding footage to the timeline. Each style of editing can be accomplished from the Source Monitor or by dragging footage from the Project Window to the Timeline.

Insert Editing: Here you add footage (a clip or audio) to the timeline by pushing existing media (if present) out of the way. This "pushing" is called rippling. When you do an insert edit, footage to the right of the insert point is pushed further forward in time. If you have marked your In and Out points in the Source Monitor, you can perform an insert edit with the comma key as a shortcut (,).

Overlay Editing: This does exactly what it's title infers. You add media on top of, and thus replacing, existing media. If the media you are overlaying is shorter than the media you are replacing, you will see the remaining portion of the original media that is left over. Overlay editing does not change the time placement of existing media. If you have marked your In and Out points in the Source Monitor, you can perform an overlay edit with the period key as a shortcut (.).