Both are Nuclear weapons that are used through the Nuclear Fission. The fissile Uranium is the U-235 the Fissile Plutonium is the PU-242.
Uranium has the 'ease' of being exploited by both shot and implosion. Plutonium can only be exploited by implosion.
Detonation by firing consists of a piece of fissile uranium that is fired at a target of fissile uranium. In this way a critical mass is generated when the explosion is combined. This method is not feasible for plutonium since, due to premature firing, contamination of Plutonium is generated due to its time constant for critical fission being less than that of U-235.
The implosion method is used by Plutonium (and it can be used by uranium or a combination of both). Basically, plutium is surrounded by a mass of explosives that, when exploded, compress the mass, generating the critical mass generated by the explosion.
Therefore the plutonium bomb is more difficult to build.