A hybrid of the tangerine and the grapefruit, or so seems to be the prevailing opinion. The tangelo comes in several different varieties, some of which may also contain strains of orange or some other citrus fruit. Most are grown in Florida.
All that's not really important unless you're a botanist, though. If you're reading this, you probably want to know what it tastes like. Well, in a word,
The best (IMHO) kind of tangelo, the Minneola, is somewhere between an orange and tangerine in size. It's deliciously sweet, not as tart as most citrus fruit, and as juicy as its grapefruit parent, well-known for squirting unwary eaters in the eye. It typically contains few, if any seeds. When in season, tangelos can be found very cheaply at roadside stands in Florida. Elsewhere, they tend to be rather pricey.