An isometric display is a way of rendering 3D graphics without resorting to the usual level of 3D transformations. Syndicate, Sim City, Ultima Online and a few other games use it. Generally you have a view from above and slightly south of the world. The advantages to this type of view are: its pretty fast (with well-drawn tiles you can probably get more detail in than an actual 3D engine, although as 3D cards get better thats less true), you don't need to Z-buffer because the render loop will remove hidden surfaces (by drawing on top of them), the math is generally easier. The downside is that its not real 3D, and things can get messy really quickly as you try to add more and more features and end up spending more and more time working out how to fake things. Technically, an isometric projection can be 1 of 3 things:

1) a 2D engine with clever graphics that make it look 3D (the most common)

2) A 3D engine with a locked camera position (more common on consoles than PCs)

3) A hybrid of the two.

I recommend this page for a good description of Isometric engines, and how to write them: http://www2.aros.net/~npawn/tuts/isointro.html

You can also find a lot of links to (and pictures of) isometric engines on this page: http://www.multimania.com/hatti/MrY/isoxeng.html

Can some one tell me?

The "original" reason for using isometric view, I believe, is that it (as opposed to perspective view) have depth without perspective distortion. I.e. objects have the same size regardless of their distance to the viewer.

In "the old days" this was the only way to create something even remotely 3D, since real-time scaling was pretty much out of the question (and looks plain ugly ).

Today, with polygon rendering/GPU HW becomming more common, the above is less true, but the isometric view kind of sticks around. I think this is mainly due to the "atmosphere", and partly due to the increased detail (It is certainly not due to increased performance).

/Niels