Ciphers are what people normally call "codes." They're encrypted texts, usually to keep some information secret, and are often used in wars.

The plaintext is the text or message before it's encrypted.

The ciphertext is the text or message after it's encrypted.

So, if we use this simple substitution cipher...

...where "A" (plaintext) is encrypted as "N" (ciphertext) and "N" (plaintext) is encrypted as "A" (ciphertext), and so on.

Ex.: "Meet me at the bridge" would be the plaintext, while "Zrrg ng gur oevqtr" would be the ciphertext.

Cryptanalysis is the practice of codebreaking.