Super n00b question of the day: what is a shader and how do I use one?

[DividedByZero]

I've been hearing a lot about this, what the heck is a shader? (in n00b terms please)

 

Adding some more questions:

 

1. What is caulk and where/how do I use it?

2. Do I put my entire map inside one big box, and apply the sky textures to the big box?

3. I made a map, but the scale is wrong, the whole thing is to small, is there a way to just stretch everything out proportionaly? Or do I just have to have a sense of how big it is?

4. How do I make lava and water?

5. Evironmental effects like rain and fog?

6. Is there one place that answers all these questions really well so you don't have to answer them for me?

 

Thanks for your time!

 

-DBZ

[Xcom]

http://www.qeradiant.com/manual/Q3AShader_Manual/index.htm