What are Trojan Asteroids and where are they located?

A Trojan Asteroid is defined by the Swinburne University Astro page "Trojan Asteroids" as:

Asteroids sharing an orbit with a planet, but which are located at the leading (L4) and trailing (L5) Lagrangian points.

These are often divided into 'leading' and 'trailing', as per the diagram below (from the source linked above):

Even though the term Trojan asteroid is often associated with the 4800 or so that lead and follow Jupiter, it actually applies to any asteroid that displays this behaviour, associated with any planet.

There are numerous Trojans co-orbiting with Mars and Neptune. Recently, single Trojans have been found co-orbiting Uranus and even Earth.