A well sweep (often hyphenated "well-sweep") is a construction that helps lift buckets of water out of an old-fashioned 'open' well. In various parts of the world it is also called a hadoof, shaduf, dhenkli, picottah or a counterpoise-lift.
It is basically an adapted seesaw; on one end is the water bucket, and you pull down on the other end to raise up the bucket. The fulcrum may be almost anything, but it is usually a wood post or sometimes a living tree. Because the bucket is on a seesaw, not a winch, this doesn't work with deep wells; the seesaw would have to be ridiculously long to bring the bucket to the top of the well.
The longer the arm you pull on to raise the bucket, the easier it will be to lift the heavy water-filled bucket. At the same time, the longer the arm is, the higher off the ground the end of the arm will be. Because of this, some arms are 15 feet or more off the ground, and will have a rope hanging from them so that they can still be used.
Other well sweeps simply use a heavy counterweight, allowing for very short arms.
Note that a short arm will not give you the added leverage that a long arm will*. The counterweight will help with the weight of the bucket, but no more. (Or, if the counterweight is heavy enough to counter the weight of the bucket plus the water, you will have to lift the (rather heavy) counterweight up in order to force the empty bucket down into the well in the first place).
The lever is usually placed on a 'Y' branching on top of the fulcrum, which gives it freedom to rotate to the side a little. A well designed well sweep will often allow you to lift the water from the well, and then rotate the lever enough to place the bucket on the ground next to the well.
* Confused? I'm not really qualified to explain the ins and outs of the matter, but a longer arm means that you will have to move it further (by pulling it down in this case), but it will be easier to move it that distance. You're trading distance for force.