As an extremely new player (never touched Warhammer before January), I have been trying to soak up as much knowledge as possible and I am confused by the purpose and uses of chaff. I see the term pop up all over the place, most notably perhaps in phrases like "dark riders are some of the best chaff in the game." What does that mean? What qualifies as a "chaff" unit? How should I be using by chaff?
Any information, and tips and tricks to use chaff (especially Dark Riders) effectively, is much appreciated. I run them with musician as mobile crossbow shooters and flee whenever they are charged, hoping to rally on 10+ leadership. They don't seem to be doing much damage...
I have a game coming up this weekend against a guy who plays almost entirely Night Goblins and monsters and I'm planning on running my Dark Riders 8" away from as many of his units as possible to bait out Fanatics. Is that a good strategy?

Good question

Chaff is basically any expendable troop.

Uses of said expendables:

- screening your more expensive troops from shooting (to hit modifiers)
- march blocking the enemy (forcing Ld checks)
- tempting the enemy to charge your chaff -- then running away = bait and flee (thus moving his units where you want them to go)
- tempting the enemy to charge your chaff -- then sacrificing them (i.e. set up your sacrificial chaff at an angle so that the enemy unit which engages is now angled off i.e. messing up the enemy unit's pursuit or exposing the enemy unit's flanks to a counter charge) = Redirecting (see DRAICH)
- double flee (dirty movement blocking trick )
- auxilliary combat duties i.e. engaging enemy chaff units (to stop the above happening to you); engaging enemy scouts, skirmishers and war machines; supporting your main units via shooting or rear charges.

Use of chaff is really an art form. Check out the DRAICH articles on redirecting, etc and some of our old battle reports for a masterclass

Chaff usually points at a cheap unit. Such units are often employed for devious tactics, and to manipulate your opponents movement to give yourself an unfair advantage. That's what tactics are all about

Dark Riders are indeed some of the best chaff in the game because they are:
- Reasonably cheap
- Almost the fastest in the game (only flying models go faster)
- Have Fast Cavalry rules. This gives them 3 benefits over chaff without the rule: they get a vanguard move, getting them in place for turn 1, they have improved mobility after rallying which makes them incredibly useful, and they can reform freely which makes very mobile and allows you to fiddle with their footprint a lot.
- Can shoot and have enough combat power to go toe to toe with other chaff

The article that explains what they can do is posted here:
http://www.druchii.net/viewtopic.php?f=3&t=71009

But I see that the images have been lost to time. If I can find some time, I'll try to recreate them.. But if somebody wants to beat me to the punch, by all means, go for it!

Thank you very much, this is extremely helpful! I managed to use some of the tactics in my recent 1750 point game against Night Goblins and achieved my first victory! I only lost about 550 points and he was down to a single Doomdiver and 1 unit of Night Goblin Archers.
The Dark Riders march-blocked his giant scary bus unit, baited out 3 fanatics, forced his backline to wheel around to deal with them, and finally when only the musician remained, charged a Doomdiver, won combat, and destroyed it (both rider and horse hit and wounded).