An Episcopal See is the office of the chief bishop of a particular church. See comes from the Latin word sedes, meaning seat, which refers to the episcopal throne (cathedra) located in the bishop's church (cathedral) in which the bishop sits as the principal symbol of his office. See is still used in that way but only in poetic contexts.
By extension, Episcopal See also refers to the local ordinary jurisdiction of that chief bishop (the diocese or particular church) and also the governmental bodies assisting the bishop (such as the curia).
With the four different meanings, things become rather confusing when sedes is used in canon law. An example of all four usages is the term Holy See, which in the Roman Catholic Church could mean the cathedra in the Basilica of St. John Lateran, the person who sits in that cathedra (the Pope), the governmental bodies which assist him in the exercise of his office (principally the Roman Curia), or the territory over which he is local ordinary (Rome), all depending on the context. This is not a problem when the Pope and the Roman Curia are both in Rome, but this presents a problem when they are in different places. Canon law eventually came up with a solution for this problem (see the article Holy See.)
Usage of the term is most common in the Roman Catholic Church and the Anglican/Episcopal churches, but is also used by some Lutheran churches; the term is also found translated into several other languages.