A conclusion to an essay is a great opportunity to do two things: 1) to tie up your argument and remind your reader of your thesis with a clear conclusion, and 2) to show why the argument matters or suggest future directions for research. First, show how everything you have written fits together and supports the claim you started with. Then, demonstrate the implications of what your research is. For example, show the possible consequences of your argument. If your paper is on pollution, talk about the impact of your argument on future legislation. Or, suggest new avenues of research your argument opens up. What possible research questions might you or others explore in the future? All this shows your reader the journey you've taken them on and gets their mind thinking about where they could take your work.
I agree with what Michael has to say about this. It is always important to tie your thesis into your conclusion but make sure you don't ever add any new information to your conclusion. If you do this, your reader looks for more and the effect on a conclusion is lost!
A simple way to write a conclusion is to invert your introduction and rephrase most of the ideas. However, because the conclusion occurs at the end of an essay you need to sum up ideas rather than introduced. Though the content should be very similar.