Actually, TNG and DS9 were syndicated, you saw it on whatever station bought the rights. That wasn't so bad for TNG, because the local Fox station aired every single episode on the same day and time. DS9 kind of got screwed by being aired late on Saturday nights on another station after the football games, meaning you couldn't really set the vcr because you might just end up with the 4th quarter of a game. It also got hurt by some pretty dull episodes in the first couple of seasons that initially turned a lot of viewers, including me, off.
Voyager and Enterprise were UPN. Voyager, like TNG, stayed on the same channel on the same day at the same time, and lasted, even though it wasn't as good a show. Enterprise stayed on the same channel, but was constantly moved around, and thanks to B&B tinkering, never lived up to its potential.