What is a stereoisomer?

Stereoisomers are molecules with the same structural formula (same number of each type of atom bonded together in the same order e.g. carbon bonded to carbon bonded to oxygen) but with the atoms arranged differently in space. It can be difficult to see how they differ from one another- try imagining a right glove and a left glove- they are mirror images of each other but are NOT the same- they cannot be superimposed upon each other.