Handball (also known as team handball, Olympic handball or European handball) is a team sport in which two teams of seven players each (six outfield players and a goalkeeper) pass a ball to throw it into the goal of the other team.
The team with the most goals after two periods of 30 minutes wins.
Modern handball is usually played indoors, but outdoor variants exist in the forms of field handball and Czech handball (which were more common in the past) and beach handball (also called sandball). American handball and Gaelic handball are completely different sports to team handball. (Read more).
The official Summer Olympics website provides more information on the game of Handball (link).