This information could be a few bits of the key, some information about the form of the plaintext, and so forth. An algorithm is unconditionally secure if, no matter how much ciphertext a cryptanalyst has, there is not enough information to recover the plaintext. In point of fact, only a one-time pad is unbreakable given infinite resources. All other cryptosystems are breakable in a ciphertext-only attack, simply by trying every possible key one by one and checking whether the resulting plaintext is meaningful. This is called a brute-force attack.
Cryptography is more concerned with cryptosystems that are computationally infeasible to break.
Steganography Steganography serves to hide secret messages in other messages, such that the secret's very existence is concealed. Historical tricks include invisible inks, tiny pin punctures on selected characters, minute differences between handwritten characters, pencil marks on typewritten characters, grilles which cover most of the message except for a few characters, and so on. More recently, people are hiding secret messages in graphic images.