there are two principal plies
For proper plie posture: the back should be flat, wide, and lengthened. The abdomen should be taught, especially the lower abs. The hips, shoulders, and heels should all be directly in line. The hips should also not be rotated forward or backward, but like an elongation of the spine. The correct feeling is a lengthened and broad back.
The Demi-plie is a half-bending of the knees without lifting the heels from the floor. This means if standing in a first position (heels even and directly under the hip sockets with the feet turned out towards the corners of a room as if facing a wall) the knees bend until the heels start to lift. The hips keep the same rotation as the feet and the knees stay directly in line with the feet. Normally the knee cap is above the end of the toes when a pull begins at the heels. However in second position the heels don't feel this pull because they are about shoulder length apart. A demi-plie in second position stops when the knee cap begins to travel past the end of the toes.
The other plie is a grand plie ( pronounced "gran"). It is a full bending of the knees until only the balls of the feet remain on the floor. The only exception is in second position where the feet are positioned the same as in a first position but the heels are shoulder length apart. In second position a grand plie passes through the demi-plie but the heels don't leave the ground. The knees bend in second position until massive amounts of pain begin (for me).
In any plie the thought of lengthening the spine and control should be asserted. Plies require balance and strength even though they are simple movements. When done correctly and with control they should feel like performing the squat weight lift. A real valuable exercise for toning the quadriceps and glutes by using your body weight and little stress. Plus flexibility only increases.