Dinosaurs were a form of life that evolved on the Earth at some point, we're talking hundreds of thousands of years ago (ish). Some of them were really big, although others were quite small. They co-existed with other forms of life, such as plants, fish and insects.
At some point they all disappeared though. Some say it got too hot for them, some say it got too cold, some say that an asteroid killed them all by changing the Earth's climate but let's face it, we don't really know.