A hypothesis is a proposed explanation, an educated guess as to why something observable happens. It is untested by definition, and therefore highly subject to revision or dismissal. For example, I might notice a pair of nesting owls outside my cabin window, and they are there every day until one of them dies. My hypothesis might be that owls mate for life. This is not an unreasonable guess on my part, based on observation, but it has not been proven one way or the other yet.
A theory, on the other hand, involves something that has been tested repeatedly over time, so that the observable results are generally agreed upon by the research community. A theory is different than scientific fact, however, as a theory is still open to change as more and more research is accumulated. A hypothesis, then, is an unproven theory.