Jesus was a man who lived about two thousand years ago.  We know about him through the Bible, which contains first-hand accounts of what he did and taught.  Jesus was also mentioned outside the Bible by historians of the time, so we know he actually existed.

But we don't just believe Jesus was a man.  We believe Jesus is God.  We believe Jesus was fully God and fully human (it's hard to imagine, but that's where the faith comes in!).  Not only did Jesus do a lot of teaching, he did something no one expected - he gave up his own life.  Since he was God, and God is perfect and without sin, he took our sins when he died.  All of them.  Through Jesus' death, and later his coming back to life, we are able to find release from our sins.