Rosin is a solid form of resin, the sticky substance that comes from trees that is not unlike sap. But while sap is used to make syrup and medicines, resin is used to make things like varnish and glazes.
Violin rosin is made by heating fresh liquid resin, until it becomes solid. It smells a bit like pine and has a glassy, orange look.
It also has a very brittle texture, which means that as soon as you accidentally drop it onto a hard floor, it can shatter like glass (the bugbear of every clumsy string player).