A folding bike is a bike that has a hinged frame, which is usually lockable.  These hinges, or joints, allow the bike to be folded into a compact unit for easy storage and transport.

 

Folding bikes are extremely popular in the UK and throughout Asia.  For some reason, they haven't quite caught on as well in the US.  (But with continued increases in fuel costs, this will likely change.) The major advantage of a folding bike is that it can easily be carried on buses and trains and into schools and office buildings. This is great because you don't have to worry about leaving a folding bike outside since bikes left unattended tend to be a target for thieves.

 

The only downside is that folding bikes often cost more than standard bike models due to the complexity of their design and manufacturing requirements. Some of the cheapest folding bikes  (by cheap I mean inexpensive, not inferior) available online can be found here on this site.