Reply #3: Typically, AE Lock is "Automatic Exposure" lock.

[ET Awful]

It will lock the exposure to it's current setting, allowing you to recompose without the aperture or shutter speed changing (depending on the mode you have the camera in).

Sometimes those buttons are multipurpose though (for instance the D200 and D300 have a button that is labeled AE-L/AF-L and you can change the function from within the menue.

NOTE: ON MOST SLR cameras, if you push the shutter release button halfway, it will focus, then, keeping the button halfway pressed, recompose your shot, then press the button the rest of the way to activate the shutter. This will give you the focus on the original target but allow you to move that target within the frame.