The main difference is that Sketchup is a tool meant for modeling structures for Google Earth program.
As SolidWorks and other mechanical CAD programs are professional tools for engineering. With these tools you can create structural frames, sheet metal parts, parts with very complex geometry, make drawings that are associative with the model from what the drawing is generated from. Then assemble those parts, run simulations for mechanical strengths, aerodynamics, thermal problems. Use CAD data for CNC manufacturing etc.
Some examples what I have done in SolidWorks:
Drawing of Roadsters base frame.
Drawing for my coworkers AC Cobra replica scuttle and firewall area
And those panels (flat patterns CNC punched and pressed then CNC bended) trial fitted
Pic 1 Pic 2
Looking quite good, don't they? Can you do those things in Sketchup?
Madis