Everybody loves you until you become competition!
Same is the case with adversaries (or call them attackers), who basically focus on preventing the users of cryptosystem from accomplishing their goals (primarily privacy, integrity, and availability of data). In order to keep your communication secure from such adversaries, a technique called Cryptography or cryptology is practiced.
Cryptology is an art of securing data by encrypting it into an unreadable format, called cipher text. Only those who possess a secret key, or say it a private key can decipher such code into readable format. Though encrypted messages can at times be broken by codebreaking techniques referred as cryptanalysis, but Modern cryptography techniques are virtually unbreakable. Modern cryptography is heavily established on mathematical theory and computer science practice; cryptographic algorithms are designed around computational hardness assumptions, making such algorithms hard to break in practice by any adversary.
Cryptography is exercised to prevent e-mail messages, credit card information, computer passwords and even with online shopping.
Symmetric-key cryptography, an encryption method where both the sender and the receiver share the same key. Under this method, receiver can decipher the hidden message only if he has the same key that sender carries too. This was the only kind of encryption publicly known until June 1976.
US government designated cryptography standards like Data Encryption Standard (DES) and the Advanced Encryption Standard (AES) to exercise across a wide range of application, from ATM cards to e-mail privacy. Advanced Encryption Standard (AES) is a widely used one.
In case of public-key cryptosystems, the public key is used for encryption, while the private or secret key is used for decryption. In public-key cryptosystems, the public key may be freely distributed, while its paired private key must remain secret. Public key cryptography can also be utilized for implementing digital signature scheme, reminiscent of an ordinary signature.