I want to run a 100 amp sub panel from my garage about 60 feet away, do I have to run it through PVC? AND/OR what size wire should I use?
You can either run it through conduit, or use multiconductor cable rated for indoor, outdoor, or direct burial depending on your route.
If you are going underground, you might consider Rigid conduit, while expensive and needing to be threaded like pipe, it only needs 6" of dirt above it rather than 18" like PVC or 24" like direct burial.
If you are indoors, any metallic conduit will provide a grounding path, sparing you the need to pull a ground wire.
For 100A we generally recommend #1 aluminum, or #3 copper if you own a copper mine. The lugs on breaker and subpanel will be aluminum, so aluminum is a good choice for heavy wires like feeders.