In the broadest of broad definitions, I would say:
Town use their votes to try and gain information and/or place pressure on the scum with the end goal of eliminating scum (this definition I feel covers basically every playstyle and philosophy surrounding votes but I admit it could be imperfect);
Scum use their votes to try and manipulate the town's perception of the gamestate and the information within to make things more favorable for them. (This definition I feel covers every scum use of their vote but I admit is very very broad. Bussing is manipulating the town's perception of the gamestate to make the town be less inclined to think two individuals are partnered together; self-voting is often manipulating the town's perception of the gamestate to make the town be less inclined to think the individual is scum; hammering oneself as scum to deny the town information is a manipulation of the gamestate and information within by denying the town that info; voting in general for scum is a tool they use to attempt to make the town believe the scum doing the voting isn't scum except in situations where it's meant to make the town believe the voted isn't scum per the above on bussing; voting town in general for scum is a tool they use to attempt to manipulate the town's perception of the gamestate and the information within to make a town elimination more likely.)
But while these broad definitions may be close to universal, obviously the specifics differ and there's far more nuance when you get down to the details which involve a lot more, so to speak, guesswork.