This page contains a brief description of the parts of a typical sailing dinghy. Most boats have parts unique to their make and class but only the more standard parts are described here. See here for a labelled diagram of a Topper.
Hull: The boat itself. The hull keeps the water out! Most fibreglass dinghies have double hulls.
Cockpit: The "hole" where the crew sits. In dinghies everything (sheets, yards, etc.) is accessible from the cockpit.
Transom: The (usually) flat vertical part at the stern of the boat. The rudder is attached to the tramsom on most dinghies.
Bow: The front end of the boat.
Stern: The back end of the boat.
Starboard: Looking forward, the right side of the boat.
Port: Looking forward, the left side of the boat.
Mast: That big stick standing vertically. C'mon, I don't have to explain this.
Forestay: For a mast standing alone to have adequate strength, it would need to be heavy. Much better to have a light mast with supports. Some bigger dinghies have wire ropes (usually) from the top of the mast to the bow. This is a forestay.
Shrouds: Same as a forestay, except to the port and starboard, and slightly aft of the the mast
Boom: The horizontal spar to which the clew corner (and on some boats, the whole foot) of the mainsail is attached. The boom is used to maintain the sail shape. The boom is attached to the mast close to the base of the mast and pivots about the mast depending on point of sail.
Kicker: A tensile member attached at one end to the base of the mast and at the other end to the boom, usually about a quarter way along the boom's length. The kicker keeps the boom from rising and so maintains sail shape.
Parts of a Sail: Most sails on sport dinghies are triangular. The leading edge of the sail is called the Luff. The base edge (along the boom) is the Foot. The trailing edge is the Leech. The corner at the top (between the luff and the leech) is called the Head Corner. The lower leading corner (between the luff and the foot) is the Tack Corner. The lower trailing corner (between the leech and the foot) is the Clew Corner.
Jib: A forward sail on some dinghies, the jib is attached to the forestay.
Jib Halyard: The rope used to raise the jib; attached to the head corner and runs down the mast.
Jib Downhaul (??): Used to tie down the jib; attached to the tack corner and the bow.
Jibsheets: A rope from the clew corner of the jib. There are two jibsheets; one to port and one to starboard. The sheets maintain jib sail shape. The port jibsheet is used on a starboard tack with the starboard jibsheet slack. Vice versa on a port tack.
Mainsail: The ...er... well; main sail of a boat. Usually the biggest sail on a boat (spinnakers and genoas are bigger).
Halyard: The rope used to raise the mainsail; attached to the head corner and runs down the mast.
Downhaul: Used to tie down the mainsail; attached to the tack corner and the base of the mast.
Outhaul: The outhaul holds the clew corner of the mainsail at a set distance along the boom.
Clew Strap: Holds the clew corner down to the boom.
Mainsheet: A rope from the end of the boom. The mainsheet is used to control the sail position - it is taken in on a beat and let out on a run.
Traveller (Horse): The mainsheet runs from the end of the boom around a block and into the helmsman's hands. This block is attached to and moves along a rope running abeam. This rope is called a traveller.
Rudder: Should I? Okay, then. The rudder is used to steer the boat. Attached to the transom on dinghies (all those that I know anyway). On most dinghies the rudder can be raised to allow easier launching and ... er... "de-launching" (!?!).
Tiller: Use this to control the rudder. It's the handle.
Tiller Extension: An extension to the tiller. The tiller extension usually pivots about the inboard end of the tiller on horizontal and vertical axes. It allows the helmsman to steer while hiking out.
Centreboard: The "fin" which protrudes from the bottom of the hull. The centreboard prevents sideways drift. Centreboards are pivotted about a pin and can be raised or lowered as required. Used in larger dinghies and some small yachts.
Daggerboard: The daggerboard is a centreboard which, instead of pivotting about a pin, is raised or lowered vertically. Used mainly in smaller dinghies.
Toe straps: There are straps attached at both ends to the deck of the cockpit. Hook your feet under these when hiking out - if you don't you'll fall out.
And to be really pedantic...
Self bailer: In some dinghies you'll find a handy little device in the deck of the cockpit which can be used to get rid of water in the cockpit. Self bailers only work when the boat is moving at considerable speed - when you're not moving they're just a hole in the boat. They also have an annoying habit of breaking, in which case they should be kept locked shut.
Bung: Most boats are double-hulled. In most dinghies you'll find a bung (usually in the transom). A bung is just a hole in the outer hull with a sealing cap, which can be opened to empty out any water which somehow managed to get into the gap between the hulls.