TGV: Train à Grand Vitesse (High Speed Train).  Besides the three generations of French TGV, this term also refers to the Eurostar, Spanish and Korean TGVs as well.
First Generation: This is the first type of TGV, from 1981.  Different from the similar second generation by having a less aerodynamic roofline and having the first truck on each end passenger car powered.  Used for PSE and La Poste.
Second Generation: Introduced in 1989, this TGV has a more streamlined roofline along with other minor modifications, and is used for the 10-car Atlantique, the 8-car Réseau and the Thalys PBA.  Only the trucks on the power cars are powered, unlike the first generation.
Third Generation: This TGV Incorporates new, more aerodynamic styling, with a much rounder nose and a single central windshield.  Used for PBKA and Duplex TGV sets.

Duplex: A double-decker third-generation TGV introduced in 1996.
PSE:  Paris Sud-Est (Paris South-East).  This is the original first-generation TGV.  First painted in orange, it now is being repainted in the more modern blue and silver whenever the sets are renovated.
PBA: Paris, Brussels, Amsterdam.  Used to define the second-generation Thalys trainsets in red and silver paint that go in those cities.
PBKA: Paris, Brussels, Köln (Cologne), Amsterdam.  Used to define the third-generation TGV Thalys.
La Poste: French Postal Service.  La Poste owns 3.5 TGV sets (The 0.5 having a special wheel assembly on the end of one of the middle cars) in yellow paint.  These sets have larger doors and no windows or seats.
Réseau: Réseau means "network", and is a suitable term to describe the common 8-car second generation TGV that roams systemwide.  Easily confused with the Atlantique, the Réseau has eight cars instead of ten, a slightly different roof on the power car and two middle cars (bar and second-class intermediate) are reversed.
Thalys: Thalys International, a joint venture by SNCB (Belgian National Railways) and SNCF (French National Railways) started in 1995.
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Michael Eby 2006