Rails validation model

I'm quite new to Rails, and I'm somewhat confused by the Rails
validation model, which seems to only be triggered on update or
save/create. To me, you also want validation at the form level, even
if the item is not being saved. For example, during registration you
want to take the username and password and make sure they're not empty
(or that password has certain restrictions), but during login you don't
create anything, yet want to make sure that the form fields are filled.
Can you use RoR's validation model or do you have to structure that
manually in the the controller or model as a separate method? Seems
odd but I can't seem to find any references that show how to do this