Is there a particle called a "tachyon" that can travel faster than light? If so, would Einstein's relativity be wrong? According to Einstein no particle can travel faster than light.
As a matter of fact, Einstein's theory of relativity is what predicts the possibility of tachyons (faster-than-light particles) - or rather, relativity is what makes tachyons special. Without relativity, particles moving faster than light would just be really fast particles.
Although it's often said that Einstein's relativity says that nothing can move faster than light, that's not actually true. What special relativity actually says (among many other things) is that anything which moves slower than light will always move slower than light, anything which moves at the speed of light will always move at the speed of light, and anything which moves faster than light will always move faster than light. Effectively, nothing can cross the "barrier" of light speed. So there is nothing in relativity that forbids the existence of tachyons.
However, based on relativity and other theories, we can predict certain properties that tachyons would have, if they exist, and they turn out to be rather strange. For example, their mass would have to be represented by an imaginary number. Also, a tachyon would get faster and faster as it loses energy, as opposed to normal particles, which need to gain energy to increase their speed. If you follow this sort of reasoning far enough, eventually you wind up with nonsensical results. Combined with the fact that no sign of a real tachyon has never been observed, this leads most physicists to believe that tachyons either do not exist, or are prohibited (by some as-yet-unknown law of physics) from interacting with normal matter.
If you're interested in learning more, I would second the suggestion that you check out the Wikipedia article that anna linked to in a comment.