Literally meaning "where the birds reside", torii are gateways at the entrance of Shinto shrines, or "jinja". They are typically made of wood, stone or sometimes iron. Most wooden torii ar painted in red. The best place to see torii gates is Kyoto's Fushimi Inari Taisha, where thousands of them are lined up in a tunnel-like fashion.
It sometimes happens that Buddhist temples have a torii too, like the old Shitenno-ji Temple in Osaka. These are however exceptions.