Special Relativity is based on two postulates:
The Principle of Relativity: The laws of physics are the same in all inertial reference frames. One cannot tell by any experiment whether one is at rest or moving uniformly (that is, moving in a straight line with constant velocity). In other words, there is no such thing as absolute rest. All motion or rest is only in relation to other observed objects (i.e. I can consider myself not to be moving with respect to the earth while at the same time I am moving very rapidly with respect to the sun).
The Principle of the Constancy of the Speed of Light (c= 2.99*108 m/s): The speed of light in free space has the same value c in all inertial frames.
A moving object weighs more than an object which isn't moving. It can be shown that if an object travels at the speed of light, it must either weigh nothing (which is only true for light) or it is infinitely heavy. This implies nothing can travel at the speed of light… other than photons, which are the massless ‘particles’ that make up light. Having said that a photon is really just a description of a process, the process of electrical and magnetic fields inducing each other and moving forward.
There is nothing in the theory of relativity which prevents an object from travelling faster than the speed of light, although such objects can never slow down to below the speed of light or they would violate special relativity. Any particle which travels faster than the speed of light is called a TACHYON (imaginary).
General deals with space and time, how objects interact with each other on a greater scale. It deals with problems presented by E=mc2 in more detail – Energy is mass; as well as energy’s part in gravity. Special is useful for problems in everyday life and ‘local’ interactions. Where General Relativity is needed is in the presence of heavy objects or large amounts of energy. It also deals acceleration in more detail than Special.
The special theory of relativity is concerned with relative motion between non-accelerated frames of reference. The general theory deals with general relative motion between accelerated frames of reference. In accelerated systems of reference, certain fictitious forces are observed, such as the centrifugal and Coriolis forces found in rotating systems. These are known as fictitious forces because they disappear when the observer transforms to an non-accelerated system.