First of all, you are confusing terms.
An EPISIOTOMY is a deliberate cut into the perineal body, made by a trained care provider, to make more room, or relieve a "band" of tissue that appears to prevent the baby from being able to be delivered without apparent difficulty. Use of episiotomy techniques is considered very controversial; some doctors and midwives prefer to take a "wait and see" attitude, and let a woman deliver, tearing, possibly, believing a naturally-occuring LACERATION is better for healing, in the long run.
LACERATIONS vary by degrees as follows:
First degree: Vaginal mucosa
Second degree: Vaginal fascia and perineum
Third degree: External Anal Sphincter (partial or complete)
Fourth degree: External Anal Spincter (EAS), Internal Anal Sphincter (IAS), and rectal mucosa
Most deliveries, if handled properly, and the nurse and doctor do not stretch or manipulate the perineal body excessively, and the baby's delivery is not too rapid, the resulting tear rarely exceeds first or second degree. Sometimes, for very large babies, or rapid expulsion, you will see 3rd/4th degree tears happen. Also, you may see an "extension" to 3rd/4th degree of an episiotomy occur in some cases.
Imagine this by putting a tiny tear in a piece of paper, then putting extreme pressure on it---what happens? It tears further, along the tear line you created. This is why, among some reasons, the use of episiotomy is so controversial. Increasing numbers of doctors are now deferring episiotomy for a "wait and see" approach, which I personally, like.
Hope this helps you.