While there are more than four strokes in table tennis, the rest derive from these four. They are the forehand topspin, forehand backspin, backhand topspin and the backhand backspin.
These directions are assuming you are right-handed. For left-handers it is easy to reverse the instructions. Forehand means the rightside, backhand means the left side of your body and the table.
The forehand topspin: The ball is struck with an upward and forward motion causing it to spin forward(away from you). When it lands on the opposite side of the table it will bounce fairly low and forward(depending of the amount of force used).
Make a fist of your left hand. With your right hand open as if it were a paddle, graze the upper portion of your fist(at the knuckle of the forefinger) with an upward and forward motion. That is the motion your racket should make as it contacts the ball.
The speed of the stroke can be as slow or as fast as you want it to be.
The forehand backspin (Also called a push): The ball is struck with a downward and forward motion causing it to spin toward you as it goes across the table. When it hits it bounces forward and more upward than the topspin.
Again using your hands as racket and ball, make a shoving motion with your right hand and contacting the ball at the thumb knuckle stroke downward and forward.
Again the speed can be varied. This stroke is mainly used for control and not to outright win a point. To some people this is the most important stroke in tt. This stroke is intended to be used to keep the ball in play and set up your attack.
The backhand topspin. Just as the forehand this stroke puts topspin on the ball forward and away from you.
Use your hands again and make the stroke. With your fist held directly in front of your body bring your racket back to just touching your body and stroke forward and upward contacting at the bottom knuckle of your thumb.
The backhand backspin(Also called a push)
This causes the same spin as the forehand backspin, just from the backhand side. Again, mainly used for control. Depending on the speed of the stroke and the amount of spin put on the ball.
As mentioned before, all other stroke are from these four stroke. Topspin is usually offensive and backspin is usually defensive.
These four strokes will be the only ones used in the review of designing a practice session.