Most people think of security and cryptography as something used by the military during wars to communicate without the enemy knowing or by governments to keep their secrets.

During the second world war the Germans used their Enigma machines to send messages. They thought those messages were unbreakable since Enigma applied a different code to each letter of a message. However a group of high-IQed British people managed to crack Enigma codes with the world's most advanced computers then, and managed to read Hitler's mail, which helped the Allies winning the war in Europe and Africa. 
At this same war, on the other side of the globe the United States was braking the codes of Japan with their crypthoanalysis team in Pearl Harbor. The Japanese didn't believe their codes could be broken, till the end of the war. The Japanese system was representing each word by a randomly assigned set of five digits. For example, 78934 might stand for "Tokyo", and 78935 for "Suicide". Every time they changed their codes it might be months before the US could read them again. The United States managed to crack about 25% of the Japanese messages, and it was enough. 

Julius Caesar, two thousand years earlier, when sending messages used a system of cryptography on his messages to his troops. He used to rotate each letter of the messages by a number of letters, for example, the word ATTACK would become CVVCEM (Rotating each letter 2 letters ahead). Back at those days this system was enough against the semi-literate barbarian spies of the enemy. 
Today, any 10 year old kid could crack Julius Caesar's messages, after all there are only 26 possibilities to check... 

Let's go back to the present: 

A senate subcommittee, after finding Pentagon computers are hit with 250,000 break-ins a year, will focus next on the State Department. Investigators say State, often called "Foggy Bottom", has only a foggy notion of how many outsiders regularly invade it's Internet-connected computers. The Justice Department has slowed computerization programs because of hacker attacks. 
Only one in 500 break-ins is detected and reported to the Pentagon, which has the government's most advanced hacker-awareness program. Two hackers used the Internet to gain access to an Air Force computer in Rome, N.Y. The hackers then used that connection to invade a computer system at NATO headquarters in Europe. Another one of their victims is NASA, which apparently was tapped for coded passwords that the hackers sent to an unknown location in Latvia. 
(Source: Dow Jones News Services, The Wall Street Journal, May 31, 1996. Copyright 1996 The Wall Street Journal) 

The U.S. Central Intelligence Agency web site was being broken into by some cyber graffiti artist who have made his/her mark on the home page of the CIA (http://www.odci.gov/cia) by changing it to read the Central Stupidity Agency. Another part of the message also tell the spy agency to "Stop Lying". This happened on Wednesday night September 18, 1996 and by the next day the web site was closed for repair. 

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Today, security is one of the hottest subjects. The internet is entering our lives fast, and a lot of sensitive information is passed through it. Anyone who is connected to the internet should know about security. Using the internet without some sort of protection is like leaving your door open, when you're not at home. Security leaks are present everywhere : from E-mail messages to Web pages that can harm your computer. Watch out !