(Spoilers to come.)
It's always been assumed that the only way for the Doctor to end the war was to destroy his people as well as the enemy, but at the end of season four more is revealed. As he explains in The End of Time, during the Time War the Time Lords were becoming too ruthless and too ambitious.
A quote from The End of Time:
Lord President: "We will ascend, to become creatures of consciousness alone. Free of these bodies, free of time, and cause and effect, while creation itself ceases to be."
The Doctor: " You see now? That's what they were planning, in the final days of the war... I had to stop them."
The Time Lords had been poisoned by war and ambition, and the Doctor had to stop them from going too far (as he began to do himself in The Waters of Mars) and becoming gods themselves, so he destroyed them and ended the Time War.
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The Time War.
There are male (the Doctor, the Master etc.) and female (Romana, the Rani, Susan etc.) timelords. It's pretty easy to figure out from there.
The Doctor is the last of the Timelords, from the long running BBC si-fi series "Doctor Who". The Timelords were a race from the planet Gallifrey. During the Last Great Time War, between the Daleks and the Timelords, The Daleks, Gallifrey and the rest of the Timelords were Destroyed. The Doctor was instrumental in the destruction thereby bringing the end of the War. The Doctor doesn't hate his planet, but has instead, been involved in a long debate with his people about the nature of the Timelord's place in the universe. This disagreement was the genesis for The Doctors travelling lifestyle. In 'The End of Time', we see scenes from the Time War, as the Time Lord president Rassilon prepares to shift Gallifrey itself out of the Time War, to modern day Earth. Here we learn that Time Lord society had become brutal and bloodthirsty over the course of the ceaseless war against the Daleks. Their own plan was 'the end of time itself', to destroy all of creation and for the Time Lords to ascend. This, the Doctor explained, is why he had to destroy his own people too, and never let them return.
The Toclafane's appearance is in the episodes The Sound of Drums and The Last of the Time Lords, the final two parts of the three-part season three finale in New Who.
River Song is an older version of Amy and Rory's daughter (Melody Pond) and probable love interest of the Doctor. Having been conceived in the TARDIS (the Doctor's time machine), Melody Pond/River Song received a very high exposure to the time vortex, resulting in her DNA being partly similar to that of the Time Lords. This means that River has the ability to regenerate, although she used up the rest of her regenerations to save the Doctor.