Hi! Shortly, metta is a mental state, one of the Ten Perfections (virtues cultivated to purificate yourself), but can also be referred to a form of meditation (like vipassana). To really understand what "metta" means, you may consider a symbol of metta: a mother holding a baby. This is the love that "metta" implies. A pure love, an unselfish love, the love that normally only a mother (or a father, obviously ) can feel for a child. There are pages, on Wikipedia, describing metta and the ten perfections, but I think you already read those pages. You can find a lot of information on BuddhaNet, here. This is the metta section's frontpage. Check every link, especially these.
