My kitchen guy cajoled me into under cabinet lighting, and I now love it. It took me a while to even start using, but I find it is really helpful for task lighting. Mine are on a single master switch which I generally keep in the on position at all times. I tend to keep them all turned off and then I use the individual on/off switches to turn the specific one on when I need that one for better light. Sometimes I use the master switch to turn them all off. Definitely go master switch with the on/off high/low toggle switch on each light.

We haven't installed ours yet, but we have it hardwired and will install with a dimmer switch. If you want the dimming option, you can't use fluorescents, (though I THINK there may be new dimmable ones out there?), so we are using xenon. Cuz we want to match the halogen lights we have elsewhere, but halogen is supposed to be very warm. There are also LED under cabinet lights now too-

See which kind of light you prefer when at the lighting store. Decide if you want the type of lighting you have to be consistent, ie halogen, incandescent, fluorescent, etc.
Do you want the dimming option? How thick are the fixtures?

Gosh, my guy isn't here at the moment so I can't give exact details, but mine are on a main switch, are dimmable and I love them. At night I dim them way down to provide just enough light to keep injuries at bay if someone should be wandering around past bedtime.

As for the lights themselves, unfortunately all I know is that they appear to clip on to a track, and look as though you can slide them along the track. I have 2 small rectangular shaped bulbs attached to the track under each cab. The track iself is very skinny. Sorry this isn't more helpful! The contractor is at lunch and I didn't pay enough attention when he was telling me about them!

We have Juno trac 12 undercabinet lights. It is hardwired with the transformer in the basement. We are very happy with them. You can move the lights easily - though I can't imagine needing to do it once you have them placed where you like them. (We did an initial adjustment so light wouldn't glare on the side of the cabinets.)

They are not dimmable - just something we didn't feel we needed since the recessed lights and island pendant lights are all dimmable. Plus all the undercabinet lights are on one switch by choice. We like them that way. :) Good luck!

If the walls aren't dry-walled, I'm assuming no cabinets are in place, so how will the electrician install the light fixtures?

Most likely, he'll stub out the wiring, then return after the cupboards are in place to install the fixtures.

Or am I missing something?

Do you know what type of lighting you have in mind? Low-voltage, line voltage, fluorescent, xenon, halogen, LED? If it's a low-voltage system, your electrician will need to know that because a transformer will need to be installed.