In conventional cryptography, also called secret-key or symmetric-key encryption, one key is used both for encryption and decryption.
The Data Encryption Standard or DES is a conventional algorithm once recommended by the US government for commercial use, but not for classified information. There are several modes of operation that DES can use. The US government recommended not using the Electronic Codebook or ECB mode, the simplest and weakest mode, for any confidential messages. They preferred the use of the stronger and more complex Cipher Feedback (CFB) and Cipher Block Chaining (CBC) modes.
If say a system is set to provide 40-bit encryption then it has one trillion keys available. A brute-force attack of 500,000 keys per second would take approximately 25 days to exhaust the key space combinations using a single 3 Ghz Pentium 4 computer. With a Rainbow Table, you can decrypt 40-bit encrypted files in seconds or minutes rather than days or weeks.
Standard DES which uses a 56-bit key, can threfore be broken fairly easily by an automated computer search for the key. Triple DES is essential for security. As DES can be broken it has reached the end of its life time.
Another cipher example is Rijndael. Rijndael has been adopted as the Advanced Encryption Standard - AES and uses a 128 or 256 bit key. It is a very reliable and fast cipher; if you are unsure what to use - use AES.
For a sender and recipient to communicate securely using conventional encryption, they must agree upon a key and keep it secret. The problem with conventional encryption is key distribution... Anyone who overhears or intercepts the key in transit can later read, modify, and forge all information encrypted or authenticated with that key. How do you get the key to the recipient without someone intercepting it?
The problems of key distribution are solved by public key cryptography.