Presta

Narrower and more secure than they're Schrader counterpart, Presta valves consist of a threaded or smooth valve shaft with a valve core installed in the end. There is a threaded nut on the top of the valve core which acts to open or close the system. If this locking nut is at the bottom then the valve is closed and no air will be able to enter or exit. If the locking nut is at the top of the threaded shaft then the valve is open enabling air to flow in or out with ease.

The Presta valve shaft will vary in length so that the valve is able to fit through rims with a deep profile.

For example: if the rim in question has a depth of 48mm then the valve will need to be approximately 60mm long. 48mm to extend through the rim and an additional 12mm for a pump head to securely fit onto the Presta valve head.
On the other end of the spectrum, If your rim only has a depth of 24mm and you have a 60mm Presta valve you'll have excess valve sticking out of the rim which is perfectly acceptable but not required.
If the Presta valve you have is too short to fit through your rim there are valve extenders available. A 30mm valve extender could be threaded onto a 36mm Presta valve to now make it 66mm long.

Presta to Schrader valve adapters exsist if you have a Schrader only pump, the adapter can be threaded onto the end of a Presta valve enabling a Schrader pump to be used to inflate your tires. Unfortunately there is no adapter currently available to convert from a Schrader to Presta valve.

Presta valves are commonly found pre-installed into a tube but are also available as a stand-alone valve with a rubber base which acts as a seal for tubeless tire setups.

Not all Presta valves are created equal; some have a removable valve core while other have it permanently installed. If your planning to use a stand-alone Presta valve with tubeless tires ensure the valve core is removable. In order to inject tubeless sealant into a tire it is sometimes necessary to remove the valve core and inject the sealant in through the shaft of the Presta valve. Once the sealant has been injected the valve core can be re-installed by simply threading it back into the top of the shaft.

Presta valve cores can easily be removed from the end of the Presta body if they are damaged and replaced quite easily. Valve core removal tools can be found in most automobile part stores and some bicyle shops.