When a re-make of something is created without giving credit to the original. Also, when something is inspired to the point of blatant copying. Usually, rip-off connotates a dearth in quality, as the person or group who does the ripping-off lacks the creativity of the original creator. A very negative term.
Some examples of rip-offs:
Also, in the United Kingdom, the term rip-off refers to a "scam, con, sting, or minor theft" (thanks to wertperch for this information).