A reference frame is a particular coordinate system chosen to represent physical entities. The notion is most often used in special and general relativity to denote particular coordinates chosen on the spacetime manifold.
Reference frames are a concept most often used in special and general relativity, but they are also relevant to Galilean systems. They are essentially coordinate choices on a (spacetime) manifold.
Often they refer to a particular observer, who is chosen to be (momentarily) stationary in the spatial coordinates in the frame considered. This is then called the observer's frame of reference.
A frame in which no forces act on the observer in question, and which is time-independent as well as homogeneous and isotropic in its spatial coordinates is called an inertial reference frame. Every observer travelling with uniform velocity below the speed of light possesses such a frame.
If the observer is accelerated and therefore not uniformly moving, it is nevertheless possible to choose a so-called momentarily comoving reference frame, sometimes also called the proper frame.
This tag should be applied to questions that deal with the differing perceptions of observers in special and general relativity, as these arise from the observers using different frames of reference, often leading to situations someone not familiar with relativistic thinking might think to be paradoxical. The tag is also suitable for those cases in non-relativistic mechanics where the properties of Galilean transformations are discussed.