When playing defense there are two main strategies: one for when the person you are on has the disc, and one for when they don't.
When the person you are on has the disc, you are called the marker. Your job as a marker is not to keep them from throwing the disc. Instead, it is to block off their throws to half the field. This means the rest of the defense will know where the throws are going and will cut them off.
How do you do this? First, decide which side you will force them to throw to. This is called the force. Usually the force is the same for the whole point so people don't have to figure it out again after every throw. Then, stand on the opposite side of them. You should get nice and cozy up to them to make it harder for them to throw. You are not allowed to get closer than a disc's width. Good marker posture is to stand with your arms and legs reasonably far apart to make a big wall around them. You should have the arm in front of them somewhat low, and the arm on the other side high.
This picture shows the part of the field cut off by a good marker.
This picture shows Chucky demonstrating good marking posture.
If you are not marking, you should be standing near your person, who is probably in the stack. There are two basic rules of defense:You might note that these two rules are sometimes contradictory. Number 2 is more important if you are covering someone a lot faster than you. Then you don't mind giving them some short catches in return for not scoring on you.
When everyone is doing what they should, the field should look something like this picture.
Note that the marker is cutting off half the field, and that the stack is lined up behind the marker so as not to get in the way of anyone cutting. The defense is all standing closer to the open zone than the offense. Good team!! You get a biscuit!