The tangor is a hybrid citrus fruit, a combination of the mandarin orange and the sweet orange. Their name is a portmanteau of tangerine and orange (tangerines are hybridizations of mandarins).
Tangors are small and sweet, with thick, easy-to-peel skins and easily dividable sections. You have probably eaten one; the various varieties all have their own marketing names, and tangors have appeared on supermarket shelves under names such as the clementine, the easy peeler, the murcott, the nule, the temple orange, the tango, and the Golden Nugget. Most of these, although not all, are seedless (beware the Monreal, a variety of clementine with a surprising number of seeds).
It gets worse: because these are often brand names and not names of specific cultivars, you can buy cuties one month and get a tangor, but a month later buy the same box with the same label and get a mandarin. This is done to provide a constant supply as different types of citrus go in to and out of their fruiting season, and the brand is invested in providing you a good, consistent product regardless of the technicalities of biology, but it does mean that you are never quite sure what you are eating.