As for getting a keelboat down a ramp, smaller boats with moderate draft -- around three and a half feet/1 m -- might be able to get by, depending on the ramp, with a tongue extender on the trailer. I'm thinking up to say a Rhodes 19 or Santana 20, maybe.
Deeper draft keelboats depend upon being lowered on a rope, heavy strap, or wire cable, perhaps 50 to 80 feet long, rolling down the ramp by gravity after tension has been taken on the launch cable and the chocks have been removed from the trailer tires. A large, swing-down or bolt-on front wheel is useful in helping the trailer go down straight. Shorter, rougher, clogged, or super shallow ramps can be an issue. Many smaller racing keelboats have provision for crane launching and many yacht clubs and some marinas will have cranes that can lift 3,000 - 6,000 lb. or so boats. Somewhere way back on my or my wife's blogs are some pictures of trailer launching (desertsea and itsfiveoclocksomewhere, both on blogspot.com).
Somewhere on the US Sailing website -- it sort of sprawls and can be hard to find some things on -- there are some good introductory explanations of sailing.