Binary, octal, decimal, hex.
Converts a number between arbitrary bases (2–36). Common: binary (2), octal (8), decimal (10), hex (16).
Digits beyond 9 use letters A–Z, standard convention.
Binary and hexadecimal are the two bases that matter most in computing — binary because it maps directly to how hardware stores bits, and hex because it's a compact, human-readable shorthand for binary, with each hex digit representing exactly four bits.
255 in base 10 = FF in base 16.
Two hex digits map to 0–255 exactly, one byte each for R, G, B — compact and unambiguous.
Uses 0–9 and A–Z, sometimes used to compress numeric IDs into shorter strings.
Digital circuits are built from switches with two states (on/off), which maps naturally onto binary's two digits — every other base a computer displays to you is ultimately converted from binary under the hood.