Bump maps are textures appiled to objects to give them..well bumps. take a look, you can get some really cool results with bump maps

http://wanbridge.net/stuff/space/index.html

though a bump map gives the illusion of displacement, it is still and illusion

here is a wall with and without a bump map

http://wanbridge.net/stuff/walls.jpg

There are other texture mappings called displacement maps, these actually ..well..displace the original object

What i mean to say is that a displacement map will physically alter the shape of a geometric form

In the folowing example a turbulence displacement map is added to a flat plane to give the effect of waves, because this is a displacement an animation actually shows rolling waves :

http://www.black-and-white-to-color.com/stuff/lwocean.jpg

here is an example using an image of leaves to displace a terrain, sorry it was one of my first candles !!

http://wanbridge.net/stuff/leaves.jpg

And another version the trees are also displacements

http://wanbridge.net/stuff/forest.jpg

And here a displacement image of some clouds has been palced onto a terrain to get this deep cloud effect

http://wanbridge.net/stuff/cloudlattice.jpg

I think what all this is saying is that 3d is mostly illusional, when you forget trying to reconstruct the real world using real world techniques and get into the realms of illusion and adpatability you really start to have fun

Most of these illusions come into their fore in animations when the viewers attention is distracted and the dynamics of the scene allow for those little give aways to sneak by unnoticed

One of the sad things about getting into 3d is that you spend a lot of the time watching movies like LOTR and looking at the little details, ( "i know how they did that !!"..or "ohh that was bad i could have done it better on a 486 !!")