What is the different works of Herodotus and Thucydides?

Thucydides only described events that happened during his own lifetime, specifically the Pellopenesian War. His works are important because he was the first to pursue a 'scientific' approach, carefully researching source material, basing motives on the hard facts around them, and not bothering with any assumed interference of any gods. Because he wrote about his own lifetime, he had the luxury of being able to fairly easily verify his facts and to talk with surviving witnesses.

Herodotus wrote about history, the events that had happened before his own lifetime. He is called 'the first modern historian' because he also tried to base his stories on facts instead of on myths. But his facts were much more difficult to verify as there were often no surviving witnesses and archeology was a thing of the far future. So he had to verify 'facts' by comparing the stories about them and deciding whether they were plausible or not. Often he told his readers that he considered something 'true' because several spokesmen told him the same story; or he told his readers that - and why - he thought a story implausible, or he invited them to judge for themselves.