Original Post
Here's an article about it in case you want an in depth explanation.
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Originally Posted by Article
Bump maps are really great for creating tiny details on a model. For example pores or wrinkles on skin.
The first thing you should understand about Bump maps is that the detail they create is fake. Bump maps create the illusion of depth on the surface of a model using a very simple lighting trick. No additional resolution is added to the model as a result of a bump map..
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Royal pretty much explained what it is, I'll link you to nikosefs tutorial about all this if you'd like to use them yourself or see some more stuff about it: http://forum.toribash.com/showthread.php?t=560526
Also, this is probably a really good way to see what a difference it can make.
Also, this is probably a really good way to see what a difference it can make.