UTF-8

The dominant variable-length encoding of Unicode.

Also known as: UTF8, Unicode Transformation Format 8

UTF-8 is the dominant way to store Unicode text as bytes. It’s variable length: each character takes 1 to 4 bytes.

Characters	Bytes	Example
ASCII (U+0000-007F)	1	`A` → `41`
Latin accents, Greek, Cyrillic, Hebrew, Arabic	2	`é` → `C3 A9`
Most other scripts, including CJK	3	`中` → `E4 B8 AD`
Emoji and rare characters	4	`😀` → `F0 9F 98 80`

"é".encode("utf-8")      # b'\xc3\xa9'
b"\xc3\xa9".decode("utf-8")

Why it won

Backward compatible with ASCII. Any ASCII text is already valid UTF-8, so old files and protocols keep working.
Compact for English and code, since common characters take one byte.
Self-synchronizing. You can find character boundaries from any byte, so a corrupted byte damages only one character.
No byte-order problems (unlike UTF-16 and UTF-32).
It handles every Unicode character.

Bytes aren’t characters. len(b"...") counts bytes, and len("é") counts code points. They differ for anything non-ASCII.
Slicing bytes can cut a character in half and produce invalid data.
Always declare it: charset=utf-8 in HTTP and HTML, and encoding="utf-8" when opening files. Don’t rely on the platform default.
Invalid byte sequences raise errors when decoding. Decide whether to fail, replace (�) or ignore, rather than guessing.
A BOM (a marker at the start of a file) is allowed but usually unnecessary, and can confuse tools.
MySQL’s old utf8 stores only 3 bytes per character, so use utf8mb4 for emoji.

Use UTF-8 for files, databases, APIs and source code unless you have a reason not to.