A very common type of collecting hobby is 'collection of currency or bank notes of various countries of the world'. At this digital and plastic money (credit cards) age, still people use paper money or currency notes. Like coins and stamps these currency notes reflect basic information about that specific country.
Lots of people have dedicated their lives to researching and compiling information on paper money or bank notes.
Do you want to start collecting paper money?
The best way to start is to subscribe to Bank Note Reporter. It's a sort of monthly newspaper for paper money collectors and not only does it carry all the latest news about new issues, currency changes, etc., it also has advertising from a very large number of dealers and it lists paper money shows in the US and around the world. BNR offers free samples to anyone (not just in the USA).
The Professional Currency Dealers Association offers a small booklet called “How to Collect Paper Money” which, if I recall correctly, even contains a few sample world banknotes.
The most effective way to get paper money is not through general circulation, but through dealers, collectors, auctions, and shows. There are many online sites which can help you trade paper money or back notes.
The International Bank Note Society's quarterly journal carries free advertising “of a non-commercial nature” for people wanting to update their personal collections. Bank Note Reporter also has a lot
of small cheap ads for this. These are good places to look for people to trade notes.
Another way to buy/sell notes is through mail auctions. Some people prefer to collect notes from general circulation. This is a good way for someone to start getting interested in paper money, but it's really not a very effective way to collect it.
How to keep banknotes safely?
To keep them in the best possible condition is to use inert plastics of good quality.
Common household products such as plastic wrap, plastic bags, window envelopes, etc. should not be used for storing paper money. Most plastics contain PVC, which deteriorates with time and heat, releasing acids and gases which will migrate into the paper, resulting in the notes appearing to have been soaked in oil, or so brittle they shatter at a touch.
Notes should be stored or displayed away from direct sunlight, to avoid fading. Repairs should be made only with products that are stable.
“Magic” tape and other plastic tapes will leave permanent stains on notes.
You can also keep notes in regular paper envelopes as well, although they're more liable to get damaged in handling.
Paper money has both a market value and a personal value. These two things are very different. There are big time dealers who keep a note from circulation just because it has an interesting serial number or for other sentimental reasons. Don't let “market” forces blur the real meaning of being a collector.
* A good site with images of USA and some World notes