The firing pin is the part that pierces the primer on a cartridge, once the hammer is released. It may float freely in its channel (usually within the bolt) or be spring-loaded. Firing pins may also be on the hammer if the weapon is a revolver, as revolvers do not have bolts or striker assemblies.
A striker is a firing pin attached directly to a spring, which provides the tension necessary for the pin to pierce the primer. Striker-fired weapons do not need hammers, as the striker assembly combines both the hammer and firing pin function into one part.