A halohydrin is a molecule with both a hydroxyl group and a halide substituent. A halohydrin contains both a nucleophile (hydroxyl oxygen) and a leaving group (halide). Under basic conditions, such a molecule will undergo an intramolecular SN2 reaction (similar to Williamson ether synthesis) to form an epoxide.

Epoxides can also be formed be reaction of alkenes with peroxy acid.

Epoxides seem to be a little bit more important on the MCAT than they are in 1st year organic chemistry. I have had students mention epoxides on previous exams multiple times over the years.