J Cut:

A J cut is an editing technique in which the audio from a following scene overlaps the picture from the preceding scene, so that the audio portion of the later scene starts playing before its picture as a lead-in to the visual cut.

(Source – Wikipedia (edited))

L Cut:

An L cut is an editing technique in which the audio from preceding scene overlaps the picture from the following scene, so that the audio cuts after the picture, and continues playing over the beginning of the next scene.

(Source – Wikipedia (edited))