A Papal legate is a representative of the Pope to the nations. He is empowered on the matters of unity of the catholic faith and for the settlement of ecclesiastical matters.
The legate is appointed directly by the Pope, hence a legate is usually sent to a government, a sovereign or to a large body of believers. The authority of the Pope to appoint and choose the legates has been contested several times during the history. There are several ranks of legates, which existed in different times:
Apostolic legate
Legatus Nati
Legatus Missi
Legatus a Latere (the highest rank) - has to be a cardinal
Nuncio - an envoy with a mostly diplomatic mission - usually an titular archbishop.
Internuncio - a lower rank than Nuncio. Internuncios are also usually titular archbishops, but they are emissaries to less important countries.

Also see: CATHOLIC ENCYCLOPEDIA: Legate (http://www.newadvent.org/cathen/09118a.htm)