Think of it like this, in wc all the black area is the void, the unknown, the bleak black hellhole. Now your map has to be closed off from this void, now say you have a brush or something with a tiny gap or a massive gap which you can see the void through then this is a leak. There sometimes annoying to find, and have been many the cause a headache amongst mappers.

I sometimes use the massive block way of finding leaks, basically I make a big block put it right through a bit of the map, through top and bottom of map I mean, and cover a bit of an area, then compile see if its fixes, and if it is then I make the block smaller and keep doing that and looking around to see where it could be. its a good method to use me things. and remember to delete the block once you've found the leak, or just leave it and say its a tower of brick (joke!!)

Best way to prevent leaks is to avoid them, make your maps neat, keep compiling your map as you go, and dont use skyboxes, putting a bit skybox round your level will result in longer compile times due to more space in the level, like say the underneath will all get lighted and the rad wil take ages on compile. Also dont overlap objects, zoom right in as much as you can to see if your objects are inline.

Dont forget making something into an entity will make it unsolid so to speak, in other words if you make say a rock with nothing below it but the void, and make it a func wall then it will be classed as a leak, so you would need a normal brush underneath it.

*UPDATE* Slacky here....... I use pointfile method of finding leaks,while in game bring down console and type in "pointfile"(without the quotes),then close console and follow the dotted line(once you find the line that is)the leak error gives you the coordinates of the entity where the pointfile line starts from.well follow the line to where it penetrates the outer wall of your map.If your line falls short(not enough pointfiles)you can add -particles 60000 or whatever amount of particles you'll think you need on the command line of your compiler.If you use Nem's Batch Compiler he gives you a nifty little check box and a space to add particle amount in.