Countries around the world that make use of paper money, also referred to as banknotes, will always make use of various banknote security measures. These vary around the world, but are all intended to make it more difficult to create counterfeit money. This allows banks and retail establishments to verify that the money they receive is legitimate; customers who know what to look for can also verify that the money they have received is real. Generally, banknote security includes everything from the type of paper the money is printed on to the variety of ink used. Special features such as holograms, foil, and embedded features such as a watermark or strip that can be viewed when the banknote is held against the light also increase security.
A special type of durable paper is commonly used for banknote security, not just to ensure the money holds up when it is stuffed in a pocket or wallet, but to make it more difficult to reproduce. The inks and colors chosen for the printed bills are also some of the most basic types of banknote security; some countries will use shiny or reflective inks, and will use specific color gradients to make it more difficult to reprint them realistically, since the colors will change gradually throughout the entire bill.
Many banknotes will include portraits of notable people in the country. Another example of banknote security, then, is adding hidden features or special notations to the portraits. These features are often miniscule and virtually impossible to see with the naked eye, and therefore very difficult to reproduce. Even if these features are not included in a portrait, they are often included elsewhere on the bill.
Metallic inclusions or holograms are also incredibly difficult to reproduce, and are found more and more frequently on banknotes. These are embedded in the bill. Also embedded may be a strip that can only be seen when the bill is held up to ultraviolet light, or a watermark with a similar feature. Distinctive of embossing may also be used as a type of banknote security. In most cases, the details of exactly how these security features are produced are kept completely secret, and are only known by relevant employees in a country's treasury.
These are just a few of the many different types of banknote security used around the world. Some bills will have dozens of different security features used at once, many that customers are by design completely unaware of, and others that are more common knowledge. Combined, this makes it very difficult for people to get away with illegally manufacturing or using counterfeit bills.