Each banknote is full of security features, which make it impossible to replicate.  All banknotes, either current or old, have a collectible value with the security features, the sophisticated printing technique, its unique design, and the history or story behind them. 

1--Watermark
A watermark is put onto a banknote's paper prior to the printing process as a security device which deters counterfeiters. When held up to the light, a watermark is visible as either a gray image on a white background or vice versa. Whenever you see a blank spot on a bill, it is usually a watermark.  Additionally each bill is printed with some fluorescent images as additional security features.

2--Portrait
Many banknotes have a portrait of a dignitary on them. Often, the watermark is a duplicate of the portrait.

3--Serial Number
The serial number is perhaps one of the oldest security devices on a banknote. Using unique serial numbers on each bill, governments are able to closely track legitimate bills.

4--Issuing Authority
Usually government has the authority to issue banknotes.  In some countries, central banks are authorized by the governments to issue banknotes.  Banks are then the agency responsible for issuing currency in that country. At certain points in history there may have been many more than one issuing authority within the same country.

5--Denomination
The denomination shows how much of what type of currency the banknote is.

6--Background 
Both sides of the banknotes are printed with designated patterns.  The intricate colorful backgrounds on many banknotes make them more difficult for counterfeiters to duplicate.

7--Metallic Thread
Some sophisticated banknotes include a metallic thread woven into the paper of the money itself.  The metallic thread may be a shadowy vertical line in the banknote. But for some countries, the metallic strip is not embedded completely into the paper. At intervals it shows through as small shiny metal spots that look a bit like aluminum foil.  Metallic thread with special printing technique gives a banknote an embossed feel.

8--Microprint
Most banknotes feature microprint which is printing so small that it can barely be distinguished with the naked eye. As such, most people don't ever realize it is there. Upon inspection of a crisp uncirculated note with a magnifying glass, you will be able to see incredibly small printing designed to foil counterfeiters. There is currently no commonly available machinery or equipment (including computer scanners/printers or copying machines) capable of reproducing such microprinting.

9--Authorized Signatories
These signatures represent a stamp of approval making the currency official. Long ago, most banknotes were printed but did not become legal tender until each one was hand-signed individually by the authorized signatories. Usually at least one of the signatures is the Treasurer of the issuing authority.