What does it mean when number plates are labelled 'L' and 'P', and when should I use them?

In the UK, learner plates are displayed on the front and back of a vehicle. Learners must display red L-plates whenever they're driving to show other drivers they haven't passed their test to gain a full licence.

 

Green P-plates can be used by drivers who've just passed their driving test. Unlike L-plates, this isn't a legal requirement.