A beef cut from the bottom sirloin butt. The cut is available with or without fat (defatted). When roasted or grilled whole it is referred to as a tri-tip roast. Many times the roast will simply be cut into steaks.
The cut gained in popularity in the U.S. after the 1950's when it became a specialty in Santa Maria, California (see Wikipedia). It is not uncommon to see freshly roasted tri-tips on grills in front of grocery stores and at local farmer's markets. Most sell tri-tip sandwiches or you can take roasted pieces home for later use.
The cut is best grilled or otherwise can be quite tough.