Both forms of vinyl flooring are 100% waterproof and most are made of virgin vinyl.
The vinyl material is exactly the same across both sheet and plank vinyl flooring.
Sheet vinyl comes on a large roll, similar to linoleum flooring. It is installed in one big sheet that covers your space, with no visible seams.
Plank vinyl flooring comes in a plank style similarly to laminate flooring, and is installed plank by plank to create a natural wood look.
Vinyl plank flooring also has a sturdy fiberglass backing to it, unlike vinyl sheet flooring, which needs to be rolled out therefor making any rigidity impossible.
Installing vinyl plank flooring is very DIY friendly while sheet vinyl flooring has the tendency of curling up over time and since there is no distinction between planks