There is a lot of good advice here, but also a lot of sloppy jargon. Swing keels, centerboards and daggerboards are three (very) different things. They have one trait in common- the ability to vary the draft of the boat - but they each implement that trait differently and with different trade-offs.
The keel on a sailboat contains the primary ballast for the boat. With a Swing Keel, that keel (not surprisingly) swings. The arrangement differs boat to boat, but the basics are that a cable lead to the trailing edge of the swinging keel hauls the keel up and down (via a winch
of some kind), while the trailing top edge of the keel is affixed to the hull with a single pivot point. While retracted, some swing keels are contained completely outside the hull; some retract into a recess either partially or wholly within the hull. Either way, movement of the swing keel significantly changes the boat's center of gravity and resulting stability. Well known examples of swing keel boats are the Catalina 22 and 25.
A centerboard boat is much like a swing keel boat with one big difference: the centerboard does not contribute significantly to the boat's stability. It contains no ballast. The Oday line
of trailer sailers (22, 23, 25) embody this design concept. As one of the posters above noted, the ballast for these types of boats is in the boat itself, encapsulated in the hull in a stubby keel. The centerboard pivots in and out of a slot within this keel. Because its not very heavy, the tackle needed to move a centerboard is much less complicated and easier to maintain than its swing keel cousin. Lots of dinghys
that have no ballast beyond the weight of the crew also have centerboards.
A daggerboard is a completely different animal altogether. Daggerboards are unweighted fins that move directly up and down through the bottom of the hull. They can be as simple as the arrangement on a Sunfish, where the daggerboard has a handle on top of it that you simply grasp and pull (or push) to move the board to its desired depth. As you can guess, daggerboards are far more common on smaller dinghy
One more type of moveable keel is the "retractable keel", that has elements of both the daggerboard (moves straight up and down) and the swing keel (contains the boat's ballast). On the RK series of boats from Seaward, the daggerboards have lead bulbs on the bottom containing the ballast for the boat. Of course, Seaward has devised an ingenious power system to extend or retract the keels/daggerboard as needed. Lets you get into the shallows, but still have a high aspect keel with the ballast down as low as you would have on a fixed keel.
I have an Oday 23, and find the keel/centerboard combination to be the best compromise for me. I get the convenience of variable draft, virtually no maintenance of the mechanism, and the comfort of knowing that the ballast for my boat is encased within the hull and ain't falling off.