A flexor muscle is a muscle that decreases the angle between the two bones that meet at the joint. A flexor reflex is also called a withdrawal reflex because it is triggered by a danger stimulus that causes a limb to withdraw by triggering flexor muscles. A cross extensor reflex is related to this in that when a danger stimulus causes a flexor reflex on one side of the body, it also causes an extension reflex on the opposite side of the body to shift the body's center of gravity to maintain balance.
A stretch reflex is a type of tendon reflex. It happens when a muscle tendon gets distressed, usually from sudden contact trauma. The associated muscle involuntarily contracts to prevent damage from occurring to the tendon. The classic example of this is the doctor testing your reflexes by striking your knee with a rubber hammer. The strike causes a tendon reflex in the knee joint. The muscle contracts, and the leg gives a slight kick in response.