This is all about the wireless flash system from Nikon. Basic components are commanders and slaves, and there are lots of options.
Some camera's have on board flashes which can be used as commanders with limited (D70) or extended functionality (D200, D80), and flashes like the SB-600 and SB-800 (the ones I am using). Low end camera's do not provide the commander functionality. In such a case you will need an SB-800 or SU-800 if you want to take advantage of the wireless possibilities (this is the case for the D50 and the D40 for instance).
A very detailed description of the system and all related resources can be found here:
The SB-600 is a fantastic "basic" flash, with enough
power for most amateur users.
It includes iTTL modes, fill flash, can be used as a slave flash (address programmable) in a CLS system (creative lighting system of Nikon).
- reasonable power
- simple display (user interface is not too clear)
- displays focal length, and adapts coverage depending on focal length
- works with 4 AA batteries
- reasonably fast to recharge
- no diffuser (can be bought separately) see for instance http://www.stofen.com/index.htm
The SB-800 is a very interesting upgrade vs the SB-600. The main
- more power
- higher focal distance coverage
- better pixel based display
- ISO value, focal length and aperture displayed
- flash "depth of field" displayed
- two filters (orange and green for tungsten and TL light)
- omnibounce-like diffuser
- better user interface
- optional 5th battery
- connector for cable synch
- connector for external power
- integrated reflector card
- more orientation possibilities (among others downwards, interesting for close-ups)
- full commander mode (and slave mode of course)
- "gadget mode" (RPT) - stroboscopic effects (not so many people will ever use this)
this is now available on-camera on the D200, not possible on D70/D50 for instance
One of the best resources about the CLS system is the "Strobist" site http://strobist.blogspot.com/
Here is a picture showing the SB-800 kit and its components (filters, extra battery, foot, pouch, diffuser and reflector)
A few recent addition have the problem a bit more complicated:
Nikon has introduced a "commander only" SU-800. That unit is not flashing but can pilot slave flashes. An advantage of the SU-800 vs the SB-800 as a commander unit is that the SU-800 can communicate via infrared (no pre-flashes disturbing the subject - interesting when taking pictures of animals). Nikon also introduced small slaves you can even position on a ring on a macro lens for instance. That system is working pretty well too and can be recommended for macro. Next to that, the D200 and more recently the D80 include an extended commander mode (not as sophisticated as the SB-800 but good enough for 98% of the users) making the SB-800 less interesting (as a commander unit only). You can configure a very interesting system by combining the on board flash of such a camera as a commander with a few SB-600 units as slaves for instance and there are multiple other possibilities as well.
Another very interesting site with useful information:
The CLS system can be used in a studio setup. I have been using the SB600's and the SB-800 mounted on tripods with umbrellas (flash through a white umbrella or reflection with a gold or silver umbrella), adding reflectors... Fantastic for portrait or shooting on location. This gives you a relatively light and affordable kit with excellent results. Highly recommended if you don't want to invest and buy professional strobes.
As a conclusion, an excellent introductory book I would recommend: