We don't know a lot about Thucydides outside of what he, himself wrote. We know that he fought for Athens and was exiled for some time. We know that he spent at least part of that time in exile with people from the Spartan side of the war. I've read that there is some comparison between what he did and the embedded reporters of the Iraq war. We know that he caught the terrible plague that hit Athens early in the Peloponnesian war. He came from a wealthy family but we don't know much beyond that.
He started what is sometimes called 'scientific history'. This means that he worked very hard to understand historical events. In contrast to Herodotus, facts came before story.
His record of the very complicated Peloponnesian war is excellent, though incomplete. Historians aren't certain if he wrote it continuously through the war or sat down later and wrote in one shot. He reports things that he saw and he obviously talked to various people throughout the whole conflict.
It's impossible to say how historical writing would have developed without Thucydides but it likely would have been poorer.