Top Down View
A game with a Top Down View, also referred to as a Top Down Perspective or a birds eye view, is seen from above, making it impossible to see the horizon or the sky. The GTA series started with Grand Theft Auto, which had a top down view. This perspective was maintained in GTA London 1969 en -1961, en GTA2. GTA III was the first GTA with a 'normal' Third Person View, although the game still contains a top down view. GTA Vice City did not contain this perspective, but the series did not completely abandon the top down perspective, because it was reintroduced a last time with GTA Advance.
2D or 3D?
A common misunderstanding is that the older GTA's are 2 dimensional, but this is not the case. A top down view does not mean the game is 2D. The cities of the classic GTA's are completely 3D: You can go forward, to the side, but also up and down. The three dimensional city clearly can be seen when you open, for example, a GTA2 map in the GTA2 Map Editor, as you can see in the screenshot on the right.
The vehicles, pedestrians and objects in the top down GTA's are indeed flat images, and thus 2D. Still, the cars of GTA2 were made in 3D to produce a more realistic 2D image. You can see a 3D version of the Jefferson from GTA2 at the left.