Answer:
The space shuttle was designed to reenter the atmosphere and land. Skylab was a tube put in orbit that had air and temperature control systems. When Skykab came down it broke apart and mostly burned up in the atmosphere. The big chunks that were left fell into the south Pacific.
One was a vehicle that went up, let astronauts do stuff, then came back. The other was a work platform sent up there that was restocked and had crew rotated on/off.
