How does audio knnow how to sync with video?

[Tiribulus]

Are audio and video streams in mpeg2 files synced purely because they're the same length, they have some king of flagging sustem that matches points between the two or something else? Do other formats (mpeg, avi, vcd, divx etc.) do this the same way or differently and from one another? If it's purely a length of file method, it seems very precarious at best leaving me with little confidence that files demuxed with one program and remuxed with another can be in sync though I see this suggested all over which one would hope means it does work. Purely an "understanding" type of question. Have nothing specific to complain about.
Tiribulus

[El Torro]

Audio, just like the video, works with frames
If you have a PAL movie, witch has 25 fps, every second has 25 images. For every frame you have 1 audio frame too, witch is 1/25 second...

[Tiribulus]

Does this hold true for ntsc as well, in general, I know the framerates different? Also does it have a way to match the audio and video frames with each other or are they just stuck together and we hope that they correspond?
Thanks BTW,
Tiribulus