According to Varney's Midwifery 3rd ed.,
a first degree laceration involves the vaginal mucosa, posterior forchette and perineal skin.
a second degree tear involves the above and perineal muscles, any or all of the bulbocavernous, superficial transverse perineal, deep transverse perineal and pubococcygeus of the levator ani, also sulcus (more lateral) tears.
A third degree involves the above and the external anal sphincter (sometimes called a partial third degree laceration)
A fourth degree tear is one through the anal sphincter.