Roughly one hundred years ago, at the turn of the last century, the newtonian paradigm started to collapse. Scientists who only years before had declaimed the imminent end of Physics, were now faced with empirical data for which there was no adequate theoretical explanation.
Out of this conflict emerged two scientific revolutions: Einstein's general theory of relativity, to account for the discrepancies in planetary motion; and quantum mechanics, and later quantum field theory, to explain atomic and subatomic phenomena. It took physicists the first three quarters of the twentieth century to develop these theories to the point that they can account, in principle, for most if not all of observed phenomena. Why then the need for something else?
Part of the appeal of the newtonian paradigm was its universality. Newtonian physics seemed to account for a vast range of phenomena, from the very small to the very large. As the empirical horizons widened, it became necessary to replace newtonian physics by not one but two new theories. Furthermore, these two theories happen to be incompatible. In other words, either theory loses its predictive power whenever it becomes impossible to ignore the other. Therefore besides the purely aesthetic need to have a single fundamental physical theory, there is a very real need for a theory which explains what happens at those tiny length scales at which neither quantum mechanics nor gravity can be ignored. String theory emerged in the mid-eighties as a likely candidate for such a theory.
The fundamental premise of string theory is that the basic objects in nature are not point-like, but rather string-like. Remarkably, out of this deceptively simple generalisation, one obtains a theory which does not just incorporate gauge theory, supersymmetry and gravitation in a natural and elegant way, but actually needs all three of them for its very consistency. It is precisely this fact which makes string theory such a compelling candidate for a unified theory.
Two 'revolutions' punctuate the history of string theory:
the first happened in 1984 as a result of the work of Green
and Schwarz on anomaly cancellation, the second was sparked in
1994 by the work of Seiberg and Witten on supersymmetric gauge
theories and that of Hull and Townsend on string dualities.
(The third revolution is due any day now!)
There are many links on string theory. I have tried to collect some of them here in no particular order.