The ciphertext-only attack is the case where the cryptanalyst has access only to the ciphertext. Modern cryptosystems are generally effectively immune to ciphertext-only attacks.
Pure ciphertext-only attacks are rare in practice because the analyst is often able to guess some plaintext. This converts a ciphertext-only situation into a known plaintext attack; see next section. Even if he cannot guess anything, he will often have some general knowledge about the material; for example, he may know what language it is in.