Think of the Taj Mahal in India and you'll know exactly what a mausoleum is: it's free-standing building (in this case not in India but on the grounds of a local cemetery), which is intended as both a monument as well as the burial location for casketed individuals. A columbarium is the same in purpose, but not in design; instead of crypt spaces large enough for a full-size casket; it features smaller niche spaces, large enough for one (or maybe two) cremation urns.