Over the last few years, the word "hominin" has crept into the public news stories about our human ancestors. This is not a misspelling for hominid; this reflects an evolutionary change in the understanding of what it means to be human.
Up until the 1980s, paleoanthropologists generally followed the taxonomic system followed by the 18th century scientist Carl Linnaeus, when they spoke of the various species of humans. The family of Hominoids included the subfamily of Hominids (humans and their ancestors) and Anthropoids (chimps, gorillas, and orangutans). The problem is, recent molecular studies show that humans, chimps and gorillas are closer to one another than orangutans. So, scientists split the Hominoids into two subfamilies: Ponginae (orangutans) and Homininae (humans and their ancestors, and chimps and gorillas). But, we still need a way to discuss humans and their ancestors as a separate group, so researchers have proposed a further breakdown of the Homininae subfamily, to include Hominini (humans and their ancestors), Panini (chimps), and Gorillini (gorillas).
So, roughly speaking, a Hominin is what we used to call a Hominid; a creature that paleoanthropologists have agreed is human or a human ancestor. These include all of the Homo species (Homo sapiens, H. ergaster, H. rudolfensis), all of the Australopithecines (Australopithecus africanus, A. boisei, etc.) and other ancient forms like Paranthropus and Ardipithecus.
This definition is part of the About.com Guide to the Paleolithic.