N-acetyl Cysteine has a nitrogen on the molecule

L-acetyl cysteine has not.

NAC is how it is typically abbreviated. NAC is a precursor given to improve glutathione synthesis in the body. You may see NAC as N-acetyl-l-cysteine which is its chemical name. Not all places put the -L- in there when the N is there.

The L- when used with amino acids denotes its stereo left or right handedness. Biological human systems require L. Some places don't always put the L in there, as it is assumed these days to be that form which is the only active form.

Don't confuse this with acetyl-l- carnitine This is a different thing all together.