Wall studs are wood structures that look like posts. Exterior finishes are inserted onto these studs. They are found in between walls.
Floor System
Floor systems are important because they hold up the floor above. They can usually not be seen because of ceiling finishes. They can be seen sometimes in unfinished basements
Roof System
A roof system is important to hold up the roof structure. They are important if you don't want your whole roof to collapse. They are found in attics usually.