Puppy Linux is an extremely small Linux operating system that can be booted from a CD or USB stick and runs as a "live" operating system, which means that nothing is installed to the computer and no files are touched. It can also be installed directly to your hard disk. The small size of Puppy Linux means it loads very quickly, it also means that if you carry it on a USB stick you still have plenty of space for your other files and documents. The compact size means it can be loaded completely into memory, giving much faster operation as nothing has to be loaded from the CD or USB stick once it is running. Alternatively, you can run Puppy Linux on old hardware with limited processors and memory and it is often used to turn an old, otherwise redundant PC into a server for a small office or home network.
Despite the small size, Puppy Linux includes a wide range of applications: word processors, spreadsheets, Internet browsers, games, image editors and many utilities. There are also modules, called dotpets, that can be installed to expand the range of software. There is also a full GUI and Puppy is designed to be friendly to Linux newbies. There is plenty of documentation and help available through the Puppy Linux community forums and on the Wiki.
A useful tool to have around, whether for an emergency or to make good use of that old PC.