They recognise the faces, voices, unique gait, posture and body language of people from their local area, and can remember an average of around 30 people. Some have been known to recognise up to 100 different people. They remember people for around 5 years. They'll know who lives where, what your regular schedule is, and which path you regularly travel along. They even know all the local cats and dogs and even can identify specific bicycles, cars and bright clothing uniforms (such as postal uniform).
During breeding season, they are most famous for swooping anyone near their nest who they deem as a threat (about 10% of males swoop). They will avoid swooping people they recognise though, so friends get a free pass.
They also hold grudges, so if you do something to annoy or threaten them, they will remember you. Some people have been on a magpie's "enemies list", and returned to a location 10+ years later and immediately been targeted, as the bird recognised them. Some birds even teach their young which people are to be treated as enemies.