Decryption

Decryption is a type of puzzle extraction method, often turning not-letters into letters, sometimes turning letters into different letters, and very rarely turning non-letters into different non-letters.

Codes and ciphers are the heart of decryption-based extractions, but not everything that falls into this category is something invented for the purpose of obscuring information. Some things we consider to be decryptions are just communication methods for specific circumstances that aren't common enough for the majority of people to be fluent in them. As such, most decryption extraction methods can be described as a code (encryption that affects words and meanings, often replacing entire words with visuals, sounds, or other words), a cipher (encryption that affects individual letters), or a communication (system invented to optimize particular types of communication rather than hide meaning).

Codes

 * Book Code - Numbers indicating a combination of chapter, page, paragraph, line, word, and/or letter in a particular book. All you need to do is find the right book.
 * Commercial Code - Words, phrases, and entire sentences reduced to single "words" (note: not always actual words).

Ciphers

 * Alphanumeric, a.k.a. A1Z26 - The representation of letters as number based on their position in the alphabet. Example: CIPHER --> 3-9-16-8-5-18
 * Caesar Cipher - The whole alphabet gets moved forward by a number of spaces (often 13). Example: ABC --> NOP
 * Pigpen - A picture-based cipher, changing letters into angular shapes with or without dots in the middle of them.
 * Dancing Men - Invented for a Sherlock Holmes story, it replaces letters with stick figures in various poses.

Communications

 * Morse Code - Dots and dashes! The original telegraph cipher, it's been in and out of use since the 1830s.
 * Flag Semaphore - Holding flags at particular angles to send messages over long distances at sea.
 * Braille - Arrangements of raised bumps to convey information to the visually-impaired.
 * Maritime flags - Colorful flag designs representing letters and digits, commonly attached to ship masts.
 * American Sign Language (ASL) - Intended as a method of communication of letters, words, and phrases for deaf individuals, it's gained popularity as just a useful skill for anyone to have.
 * Binary - An integral part of computer programming, binary represents numbers as a series of 0s and 1s, which means it works well in conjunction with an alphanumeric cipher.
 * ASCII - Numerical code used by computers to represent letters, digits, spaces and punctuation marks. Considered obsolete for real use (no support for non-English languages) but alive and well in hunt puzzles.
 * Unicode - Numerical code used by computers to represent all sorts of letters, digits, symbols and squiggles from every language under the sun. (Even emoji!)