Is DVD-Video a worldwide standard? Does it work with NTSC and PAL?

Yes, DVD Video is a worldwide standard. It works with NTSC both PAL, however you need to be aware of the following rules:

NTSC players usually won't play PAL discs. PAL players do play NTSC discs, but some older TVs will only display a black-and-white image. Most western European TVs will be fine, however.

This means that most titles are authored separately for PAL and NTSC titles for maximum compatibility. Alternatively, since NTSC discs will play almost anywhere in the world, a single NTSC version can be produced, with a warning or disclaimer on the packaging for the few cases where incompatibilites may occour.