they are for quickly moving your mouse pointer into a corner with a pre-defined action that you set up in System Prefs. basically instead of triggering Expose with a keyboard command you can do it by moving the pointer into the corner. i love them and can't live without 'em.
If you look at my Expose preference pane, you can see how I use hot corners. Just move my mouse into the corner to activate the features in the pull down menus.
I find the top ones far more likely to be activted accidentally than the bottom ones, especially on my 14" iBook. I only use bottom left to toggle the desktop. Couldn't live without that one.
Expose, dashoard and quicksilver are why macs will always be superior to pc's. These functions just make everything natural, my life would be so much more complicated and laborious without them.
I've got screensaver on top left, expose (F9) on top right, desktop (F11) on bottom right and dashboard on bottom left, I didn't use hot corners the first few months, but now I couldn't live withouth them
Top left is nothing. Top right is desktop, because I usually open the Finder and it's right there. Bottom right is Expose, and bottom left is Dashboard.
I dont seem to have any use for the corners. The side buttons on my mightymouse activate 'all windows', the clickwheel button activates 'desktop', dashboard is in the dock and I dont use a screensaver.
I used to use them when I took my laptop out and didnt have a mouse though.
I use command and touch a side of my screen to flip spaces in virtuedesktops though.
Top left is screensaver, top right is all windows, bottom right is desktop, and bottom left is dashboard. My iBook wouldn't be nearly as usable with out my hot corners turned on.