Contact Host allows you to send an enquiry to the host and is in no way a booking request. You should always do this before committing to a booking to ensure the place is actually available.
Request to Book commits you to a booking and your card is charged there and then, regardless of if the host accepts your request. It then takes up to 7 days to get your money back if the host declines - pretty frustrating.
Instant Book means the host can't decline your booking without their search ranking score being negatively affected.