Thor-Agena was a series of orbital launch vehicles. The rockets used Thor first stages and Agena second stages. They are thus cousins of the more-famous Thor-Deltas, which founded the Delta rocket family. The first attempted launch of a Thor-Agena was in January 1959. The first successful launch was on February 28, 1959, launching Discoverer 1.
Among other uses, the clandestine Corona program used Thor-Agenas from June 1959 until January 1968 to launch US military reconnaissance satellites operated by the CIA. During this program, Thor-Agena launch vehicles were used in 145 launch attempts now known to have been part of satellite surveillance programs.
Also, Alouette 1, Canada's first satellite, was launched on a Thor-Agena-B.