Crocuses do come back year after year, and are particularly hardy bulbs. They're tolerant of cold weather, and squirrels tend not to dig them up. As a result, you may have longer year-over-year success with crocus bulbs than with other types of bulbs.
Crocuses are also excellent naturalizers, meaning that they multiply and spread throughout the years once established. They multiply by corm offsets, or baby corms that develop along the sides of the bulbs. Crocus can be divided after a few years.