SGML is a way of writing a computer document so the computer can see the structure — what is a title, a chapter, a list, a note — instead of only seeing how the page looks on one screen. You put marks (tags) around pieces of text to name their job. Because the structure is named clearly, the same file can be read and shown on a different computer system without falling apart. It is an old, heavy standard from the professional publishing and document world, not something a beginner types for fun. Later web languages learned from it: HTML was built as a special use of SGML; XML was designed as a simpler relative. Students today meet HTML and XML first; SGML is the grandparent in the background. People say the letters: S-G-M-L.

SGML（標準通用標記語言）