Skylab B was a concept for a second US space station similar to Skylab that was planned to be launched by NASA for different purposes, mostly involving the Apollo-Soyuz Test Project, but was canceled due to lack of funding. Two Skylabs were made by McDonnell Douglas for the Skylab program, originally the Apollo Applications Program, in 1970, one launched in 1973 and the other put in storage and considered as Skylab B.
When the Apollo-Soyuz Test Project was created in 1972, NASA had an idea of launching the back-up Skylab station during the ASTP mission in 1975, which would involve the Apollo spacecraft docking with the Soyuz spacecraft first, performing intended operations, then heading for the Skylab B, which during the ASTP discussion was originally called the International Skylab, for a 56-90 days extended mission.
For future missions, the station, which would have then be called the Advanced Skylab, could have been expanded by the space shuttle due to enter service in 1981, after the original Skylab station re-entered in 1979, the year the shuttle was expected to make its maiden flight. At the time the idea was discussed, NASA still had two Saturn V launchers, three Saturn IB boosters, the back-up Skylab space station, three Apollo CSMs and two Lunar Modules in storage.
After the original Skylab was launched in May 1973, the plan for the Skylab B was distinctly canceled and the Apollo/Soyuz spacecraft had to use the Docking Module launched on the Apollo-Saturn IB for performing experiments in space. After Project Apollo ended and as NASA was moving to developing the space shuttle, the remaining Apollo hardware was donated to museums in 1976. In the meantime, if the Skylab B space station had been launched in the first place, it would have been the United States' second space station launched apart from the International Space Station, which is now in orbit and under construction, although it also contains different space station modules developed by other countries.