What is an Exoplanet?
An exoplanet (short for extra-solar planet) refers to planets that are orbiting stars other than our Sun. So far more than 500 have been found, through a variety of different methods, but this number is always growing! (for an up to date count see http://www.exoplanet.eu/).
The first exoplanets were found in 1992, orbiting a pulsar, a star that has reached the end of its lifespan and collapsed to form a very dense object rotating much faster than our Sun. These exoplanets would not be a very good place to visit, due to the large amounts of radiation given off by the pulsar. The first planet discovered around a more Sunlike star, 51 Peg b, would also not be a nice place to visit - although its star is more like our Sun, this exoplanet is what we call a 'Hot Jupiter', as it's a large gaseous world (like the planet Jupiter in our Solar System) and orbits very close to its host star (and so is very hot) - in fact 51 Peg b completes one orbit of its host in only 4 (Earth)days!