A dagerboard is a centerboard -- well, at least most people use the term interchangeably. In my mind though (for what that is worth), a dagerboard is a retractable centerboard. Having a centerboard that retracts make the board a little more versatile.  If you are on the water, and the wind picks up, you can simply retract the dagerboard (making the board more controlable and faster).  If the wind dies, you can just put down the board to get back up wind.

There are many beginner type boards that have centerboards that attach just like the fin -- you just screw them in.  It is not a big deal to take the centerboard out, but you obviously have to get back to land to get a screwdriver if the wind picks up.  With a little experience though, you probably get by without putting the center fin in at all.

Not to take anything away from Chris's answer (I just am much more verbose but no more informative)

Technically:
Centerboard- lowers by pivoting on an axis. Most modern (I use that term somewhat loosely) movable center fins are centerboards.

Daggerboard - lowers by sliding down its box through the hull. Rare in sailboards because retracting it leaves a good bit poking above the hull. Daggerboards are common in small dinghies (Laser, Sunfish) and occasionally larger sailboats (more often AFAIK) in high aspect higher peformance keels.

Center fin (more or less a windsurfing term)- removable, fixed center fin to be used in light air, especially with folks newer to windsurfing who need the lateral resistance or need the center of resistance more forward to correctly balance a smaller trainer/youth rig. Usually dispensed by more able sailors and  those using larger sails even in sub-planing conditions.

IMHO- for a newbie, depends which way you want to go. eg Prodigy/Discovery, etc have centerboards for decent sub-planing upwind performance. Often (I'm qualifying this, okay) used by folks anticipating using the board relatively frequently for non-planing conditions such as cruising or light-air racing Southern style. The con is some folks don't like the knob/handle of the centerboard as it can get underfoot.

Wide boards, Start, Nova, et al, are oriented to ultimately to be used as light-air planing boards. While they have okay sub-planing performance, not as good as their centerboard brethren, they become quite capable planing boards with larger fins and the center fin removed. You also get a cleaner deck with nothing underfoot.

Answer- depends which way ya wanna go.

Talon: You can listen to all this bull crude if you want but if you want a more exact answer to your query then let me tell you:

Center board is a rudder device in the middle of your board.  It is basically there to get in the way of your feet and other than that it serves no useful purpose except to line the pockets of the manufacturer with your money cause you have now bought something you never needed in the first place - ie. capitalism.

Dagger boards are, however, a totally different and unique concept.  Ever since windsurfing took off in the Carribean the manufactures have included in all windsurfing equipment the daggerboard.  This device is disguised as a part of the sailboard but in actuality the daggerboard holds a safety device known as a dagger.  The dagger is used for self defense against pirates.  The dagger board and self contained dagger have saved the lives of many a man, woman and child in these dangerous waters.  Pirateing is down by 70% according to US Customs and they attribute this decline to the dagger board and dagger.  

By all means forget the center board and get the dagger board.  I personally have run across a few wannbe pirates up at Van Pugh.  Several have trailers others just strap stuff on top of their cars.  Never the less, pirates are everywhere.  

I hope this helps.

Ho Ho Ho.

It be time I wayed inn on this. David, I think you mean Bull Krud. "Bull Crude" is another topic probably best left to the Atlanta Bullbreeding Club website.

ok then, smell ya later

cv

Well, because.  When you are in the middle of the sea and suddenly surpirsed by a priate, with his hair on fire, coming at you with ill intent, then you need that dagger because it is the only defensive tool you can use at close range.  Besides what if you reached down to get your Pelican and it few away.  Then where would you be.  Trust your manufacturer, they have been doing this for years and they know what is best for you.  

Don't be a leader be a sheep