What is Gravity?Watch
There are four fundamental forces: gravitational, electromagnetic, strong, and weak -- these are just names we've attributed to natural phenomenon we haven't yet understood.
What we do know, however, are their properties, such as the acceleration due to gravity, or the force produced when a current-carrying wire interacts with a magnetic field.
Hope this helps!
In usual "flat" space the shortest path between two points is the straight line between them, but things are more complicated in a curved space. Consider for example two points on the surface of the sphere; the shortest curve between them is an arc of great circle, that is to say the intersection of the sphere with a plane through the centre of the sphere. In GR free (i.e. not subject to any forces other than gravity) particles move along geodesics - the distance minimising curves in the spacetime. In a curved spacetime these geodesics will be curved, giving the impression that the object's path is being bent away from a straight line. This is gravity.
As time has gone on, the description of what gravity is has changed. At the moment it is thought of as a result of space and time (they are part of the same thing really, known as spacetime) curvature.
Our description of it is incomplete, we don’t know how gravity behaves on really small scales, so the answer to this question is at the moment at least, incomplete.