What simonc describes is merely one type of omiage; not even the most common one. Usually, it is expected from someone who goes on a journey to bring back gifts for their friends, family and co-workers who didn't come along; this is seen as a sort of apology for enjoying yourself while they were not. It is not optional, and often, people will approach you and actively ask for their omiage. However, the gift is not required to be anything special; in fact, many large train stations and airports in Japan have shops where you can buy the standard gifts from many different places, in case you forgot (or were too lazy) to buy them at the place you went to.