Typographical convention to refer to a divinity, as opposed to god with a little g. The rules for its use are similar to other titles in English:
- Capitalize the title 'God' at the beginning of sentences and quotations. (Seriously.)
- Capitalize the title 'God' in compound titles: Lord God Almighty, Thunder God Cid.
- Capitalize the title 'God' when used as a name: "morning walks with God". (cf. I went shopping with Mother.)
- Capitalize the title 'God' when used as a plain title: He always wanted to be God (cf. She always wanted to be President.)
In Christian tradition 'God' is invariably used as the name of the Judeo-Christian deity (instead of, for example, YHWH) and is always capitalized. Outside of Christianity there is leeway between constructs like "the God of Christianity" and "the god of Christianity". Many people would probably consider them to have slightly different meanings, and feel the second to be derogatory usage.
See also god with a little g.