Encryption is the scrambling of data so that it becomes very difficult to unscramble and interpret. Scrambled data is called ciphertext. Unscrambled data is called plaintext. Unscrambling ciphertext back to the original plaintext is called decryption.
Data encryption is inevitably performed by the use of a cryptographic algorithm and a key. The algorithm uses the key to scramble and unscramble the data. Ideally, the algorithm should be made public (so that it can be scrutinized and analyzed by the cryptographic community), while the key remains secret.
The key is fundamental to the strength of the encryption. You need the one correct key before you can decrypt the ciphertext. It follows, then, that the longer is the key, the greater is the range of possible values it could have. The range of possible values is called the keyspace. The greater the keyspace, the more difficult it is for an unauthorized person to discover the correct key.
Encryption cannot make unauthorized decryption impossible; it can merely make it improbable. With unlimited processing capacity and unlimited time available, all cryptosystems could be broken. The purpose of encryption is to make it as unlikely as possible that a ciphertext could be broken within the period of time during which the contents should remain secret.
There is an arbitrary and subjective distinction between weak and strong encryption. Strong encryption implies that it would effectively be impossible to find the key within the effective lifetime of the secret. Weak encryption implies that the key could be found with a realistic amount of processing capacity and a reasonable amount of time. The difference between weak encryption and strong encryption is thus effectively a question of processing power. As computers become more powerful and less expensive, what is now 'strong' encryption will inevitably become 'weak' encryption. Until recently, any key length above 56 bits was generally considered to be 'strong' encryption. (Note, however, that 56 bit DES was cracked during 1998.) 56 bit encryption could now be considered weak (see also, snake oil).
Many governments around the world view strong encryption with concern. The argument is that the availability of strong encryption limits their ability to monitor the messages of suspected terrorists, drug traffickers and paedophiles. There is consequently a growing movement for governments to demand a system of key escrow or key retrieval. This would ban the use of strong encryption unless either a back door is made available to Law Enforcement, or a decryption key is lodged with a Trusted Third Party (TTP). This, needless to say, has led to considerable debate between government and government agencies on the one side, and academe, civil liberty groups and the greater part of industry on the other. (See RIPA and Carnivore)