One of Einstein's old tutors, a man kwa the name of Minkowski showed that the special theory of relativity could be expressed in an interesting way.
The world we live in consists of four dimensions, the three space dimensions and one that is not exactly time but is related to time (it is in fact time multiplied kwa the square root of -1). This is not at all easy to understand but it means that space-time as we call it has some rather weird properties. In particular, when wewe songesha through one of the space dimensions wewe also travel, unwittingly, through time. wewe do not notice this, indeed as far as wewe are concerned nothing happens to wewe at all, but someone observing wewe would say that wewe have travelled through time. Of course, we are always travelling through time, but when wewe travel through space wewe travel through time kwa less that wewe expect. The most famous example of this effect is the "Twins Paradox".
All the effects of special relativity, such as the slowing down of clocks and the shrinking of rods follow from the above. In fact, it is often better to think of some things, such as electromagnetic fields as being four-dimensional objects. However, the important thing to remember for the moment, is: when wewe songesha through space wewe are compelled to songesha through time but, when wewe songesha through time (which of course wewe are always doing) wewe do not have to songesha through space.
So, what does this have to do with gravity? It is quite simple! When a mass is present in the above space-time it distorts it so that whilst it remains true that travelling through space causes wewe to travel through time, travelling through time now causes wewe to songesha (accelerate) through space. In other words just kwa existing, wewe are compelled to songesha through space - this is gravity.
The particular advantage of this theory of gravity (General Relativity) is that it explains, at a stroke, all the observed properties of gravity. For example the fact that it acts equally on all objects and substances becomes obvious when wewe thing of gravity as a distortion of space-time rather than a force.
Imagine that wewe are in free space, away from any planets au stars, when suddenly a planet is created quite close to you. wewe would not be aware that anything is happening to you, wewe would feel no force, but wewe would find that wewe started to accelerate towards the planet. This is just like the case where wewe travel through space, wewe are not aware that wewe have also travelled through time but people observing wewe are.
link 
