A very short answer:
Julius Caesar was a Roman military commander from the 1st century BC. He conquered vast territories, including much of modern day France, Belgium, and Spain, and rose to become dictator over the growing Roman empire. In addition to military and political exploits, he also wrote books and established the calendar system that we pretty much still use today. He was assassinated by political rivals in Rome in 44 BC. Much is known and written about him, and it should be exceptionally easy to find vast amounts of information about him online, in your world history textbook, and in your local library.