The switches are mortised into the door jam and are hard wired into the wall behind the switch. When you open the door it activates the switch.

Another option is an occupancy sensor switch which can be installed in place of a regular switch. With this you'd have the option of leaving the door open without the light being on if you so choose.

Texasredhead....Do you know the name of this product? I'm in Canada so not sure if it's the same but what the heck I'm looking anyway : )
Itsunclebill...Do you mean a regular light on the ceiling and just the motion switch?
Thanks for the info!

Here's just one example. I've seen them used in restrooms, storage closets, etc. in conjunction with a timer too. Pretty slick idea if you ask me.

Here is a link that might be useful: door jamb switch

Here's a motion sensor switch, although this is by no means the only one.

Here is a link that might be useful: Leviton switch

Thanks so much............I went to HD today and looked at the 2 shown above and a regular ceiling light (i really think it was for outside but the guy told me either in or out). I figured the door jam type ($11) was too hard for DH, so I paid the $33 for the motion switch.
Thanks everyone it helped me out a lot. In my head I pictured the old fashioned type of light on the ceiling (with the string) but with a motion type of "something" attached. I'm sure the HD guys woulda thought I was Loopy!

The motion ceiling light should look like a typical hampton bay ceiling fixture with a sensor instead of the ring under the glass. I had to get a few one night for a 4 unit house commons area. The HD guy looked at me like I was nuts when I asked if they had them. While he was cutting me some wire, I grabbed them and put them in the cart and he said "wow didn't know there was such a thing"

I did this a couple of years ago in my pantry as cheaply as I could, by opening up the ceiling next to the light and installing a motion detection switch (~$10) in line and aimed directly down. No need to fish wires down to the usual switch position, much easier than installing buttons to detect door opening and by aiming it down directly inside the door, it wouldn't activate until someone actually stepped inside.