Space-time is the geometric framework in which the events of nature take place. It has four dimensions: the three usual dimensions of space - length, width and depth - plus an additional dimension, that of time. An event in space-time is therefore the datum of the place where and the moment when the event occurred.
Why are space and time, which appear to be of very different natures at first glance, linked in a single entity, space-time? It is because space-time makes it possible to interpret the theories of relativity most fruitfully.
In 1907, two years after the publication of Einstein's Specific Relativity theory, Hermann Minkowski - who had taught Einstein at the Zurich Polytechnicum --demonstrated that in his former pupil’s theory there was a mathematical quantity that could be interpreted as the distance between events taking place in space and time. In Minkowski’s hands Specific Relativity became a geometric theory of space and time, with straight lines, planes, rotations and distances, as in traditional geometry.
Later it was realized that Minkowski’s space-time was flat, like the space in Euclid's two-dimensional geometry. And in 1915 Einstein showed that gravitational fields curvespace-time in such a way that the true space-time of the universe has a curved geometry. (Inventions and Discoveries)