Here are a few definitions of the stuff you're likely to run across in catalogs:

A "simple exterior set" means the setting is outside -- the front porch of a house, in a garden, etc. Neil Simon's "Proposals" is an example of that -- it all takes place in front of the main character's vacation home. "One exterior set" is the same thing.

A "unit set" means only one set is used for the entire show.

Generally in play descriptions what you see are "int." (a room), "ext." (an outdoor setting) and "unit" (one set for the whole show) as common set descriptions. So a show that has "2 int., 3 ext." would be one that has five scene changes -- two indoor sets, three outdoor sets.

Other set descriptions you might see are:

Representational set -- you can use a few pieces to suggest a setting, such as two chairs and a table to represent a restaurant, or abstract pieces. Pieces may be hauled on and off as needed.

Bare stage -- Exactly what it sounds like: no set at all, often with back backdrop. Sometimes also called "open stage."

Int. with inset -- An interior set with space set aside for a small scene that takes place elsewhere.

Various scenes -- several locations needed, often representational

Unit set, platforms -- a one-piece set using different levels

Comp. Int. -- a composite interior set, containing more than one interior location. "Belles" and "Tea and Sympathy" are examples.

Wagon -- wheeled or railed platform with a set on it which travels in a straight line across the stage surface

Platform Stage -- set using platforms to provide different levels rather than a traditional set

Cyc -- requires a cyclorama, a curtain on a curved track across the back of the stage

Scrim -- requires that some scenes be played behind a scrim, a curtain that is opaque when lit from the front and transparent when lit from behind

Area setting and staging -- uses areas of the stage set off with lighting for different locations rather than a traditional set

Drop and wings -- requires painted backdrop and side pieces. For a good example of this type of design, <a href = "http://www.ckrumlov.cz/uk/zamek/5nadvori/t_dekora.htm">check here</a>.

Great question -- hmmm, maybe I should turn this into an article!