Please note that if you scroll a little bit further down the PF list, we DO have a section devoted to Special and General Relativity. Please post all future questions related to this topic on that section.
Special Relativity - Reconciled Newton's mechanics and Maxwell's electromagnetism...but only for things in uniform motion (non accelerating reference frame). Time and space are relative. Speed of light is constant.
General Relativity - Expanded SR to include acceleration and gravity. Provides a theory of gravity (warped spacetime).