Modules
The unit of the QR code. Don't have any specific size. Size of a module should be consistent across the whole QR code.
They have two varieties light and dark
You can have various QR code sizes which are called: versions. The larger the version, the more data that can be contained inside of it.
Look up person who stored a book in QR code.
Modes
Numeric, Alphanumeric, Byte and Kanji. These are the 4 most basic.
You can also mix QR codes
Error Correction Level
This level determines how max percentage of data that can be recovered in a partial QR code reading.
There are four correction levels: L, M, Q, H
L: 7%, M: 15%, Q: 25%, H: 30%
Versions
40 possible versions from 1-40. Each version results in a QR code that is 4 modules wider and taller than the previous version.
21 x 21 modules = Version 1
177 x 177 modules = Version 40 (39 * 4 == 156)
4 * (version — 1) + 21