Space Stations tested a humans ability to live in space.  By living in the stations the country they represented, they would would have a greater understanding of the world around them.
Skylab 1: Launched on May 14, 1973. It was hit by a meteor which damaged the outside panals that protected the shuttle from suns rays. As a result the ship overheated and needed repair.  The United States after this mishap started counting the skylabs from one when the second skylab was launched.
Skylab 2: First mannned mission to space, launched May 25, 1973 and orbited the earth until June 22, 1873.  Three crew members: Joseph Kerwin, Paul J. Weitz, & Charles C. Conrad Jr.  The crew did several expirements testing earth resorces and medical studies.  Accomplishment: Doubled the previos times in space.
FACT: Skylab had 300 scientific, techonological, and medical expirements during the 171 day trip in space.  It was also used to test humans response to zero gravitiy and research the earth's resources. 
Skylab 3: Crew members entered the station on July 28, 1973 to September 25, of the same year.  The crew was compiled with commander Alan L. Bean, Pilot Jack R. Lousma, and scientist Owen K. Garriot.  They were in space 59 days and orbited the earth 858 times.  They installed a sun sheild on the shuttle and continued some maintenance.
Skylab 4: The last skylab crew to be launched Commander Gerald P. Carr, Pilot William R. Pogue and Scientist Edward G. Gibson went to space from November 16, 1973 to February 8, 1974.  The crew orbited the earth 1214 times and took pictures of the Comet Kohutek.
Information Found: thespacerace.com