Chop blocks (where two o-linemen block one d-lineman – one above the knees and the other below the knees) are illegal.
Cut blocks (below the knees blocks employed by o-linemen and the kind employed by Denver, the Texans, and a lot of other teams who attempt to exploit cutback running lanes) are not illegal as long as the block is not from behind and as long as the block occurs within 3 yards + or - from the line of scrimmage.
Illegal crackback blocks are below the waist and usually done by receivers who move back inside at the snap to block/seal. Same as a clip basically, but at or near the LOS.
Legal crackback blocks are above the waist and usually by receivers moving back inside at the snap to block/seal.