It is the representation of situations using scaled models on a table top or detailed counters in a board games. The rules allow the situation to be developed in a way that emulates what probably would have happened but not always.
The period set can range from Ancient to Modern and into the Future. It can be on land, sea or air and often is a combination.
Many people think that we only re-enact battles. Well sorry to disappoint you they are rarely used in club wargaming. What needs to be considered is that in 3 hours (an average evenings club wargaming) you have to set up the game and try to get an agreed result. It is not always easy to get some people to agree to anything. But one thing you learn quick in wargaming is compromise.