What is a Vignette and do I need one?

A vignette is a permit to use the motorway and is obligatory for any vehicle (including motorbikes) to go on the motorway.
It is not necessary if you do not go on any motorway while in Switzerland
Official Info

Where can I get one?
You can no longer get them easily outside Switzerland, although you can get them online here, and you can purchase the stickers in UK by phoning the Swiss Centre on free-phone 00800 100 20030 for information, they are available at all border crossings that are on the motorway, not however at non motorway crossings. They are also available in Switzerland at post offices, customs offices, gas stations, garages and cantonal vehicles registration offices.
They should be stuck to an irremovable part of the vehicle and visible from the front, they are designed to be destroyed if removed, you may be fined if it is not attached.
I always stuck them inside of my windscreen.

How much do they cost and how long are they valid?
They cost 40 chf (27 Euro) as of Jan 2006, and are valid for 14 months from the 1st of December previous to the 31st of January the following year. If you buy one in the middle or near the end of the year it is still only valid from the 1st of December previous to the 31st of January the following year.

Do I need an extra one for a motor-bike trailer or outfit towing a trailer?
Although you do need one for a car or lorry trailer you most definitely do NOT need one for a motorbike trailer.
Motorcycle trailers and outfits are specified as exceptions to needing a vignette.
Part g. of this official Swiss site, sorry its in French, German or Italian only, click top right to change language.
http://www.admin.ch/ch/f/rs/741_72/a3.html
Here is a translation of the relevant part

Art. 3 Exceptions to the Tax Liability
g. fixed trailers (Article 20, 3^e Al, let.e, of the O of June 19 1995  concerning the technical requirements necessary for the road vehicles [ OETV ]), trailers of motor bicycles (Article 198 OETV) and motorcycle combinations (Article 147 OETV);

Note: sometimes the vignette sellers at the border say that motorbike trailers need an extra vignette, they are incorrect and should be politely advised so, ask them to check their rule book and if possible take a print of the Exceptions to the Tax Liability page from the above website to show them.