A common misconception is that the Rocket was the first steam locomotive. In fact the first steam locomotive to run on tracks was built by Richard Trevithick 25 years earlier, but was not financially successful. George Stephenson, as well as a number of other engineers, had built steam locomotives before. Rocket was in some ways an evolution, not a revolution.
What marks the Rocket out, is that it was the first of the 'modern' locomotives, as it used a multi-tubular boiler, which made the engine much more efficient. Previous boilers consisted of a single pipe surrounded by water. Other innovations included in the design were such things as venting the exhaust steam up the chimney to pull fresh air into the fire - increasing the heat of the fire and the pressure in the boiler, making the Rocket quicker.
The design of the Rocket therefore was revolutionary, and nearly all steam locomotives built since have been based upon the Rocket's basic design.
It was designed and built to compete in the Rainhill Trials, a competition to select the locomotive type for the Liverpool and Manchester Railway in October 1829. All the other competitors broke down so a true result is a bit hard to tell; however the winning Rocket did fulfill the key requirement of the contest that a full simulated 90km round trip under load be completed with satisfactory fuel consumption. The builders of the Rocket had already built about 50 engines, and presumably were fairly good at doing this.
At the official opening of the railway almost a year later on September 15 1830 the first run of the Rocket was marred by the first railway casualty in history, with the accidental death of William Huskisson.
Stephenson's Rocket still exists and resides at the Science Museum, London in much modified form compared to its state at the Rainhill Trials. The cylinders were altered to the horizontal position, compared to the slanted arrangement as new, and the locomotive was given a proper smokebox.