A three way switch is a single pole, double throw (SPDT) switch:
The common terminal corresponds to the one on the left in the picture, the two throws on the right are the traveler terminals. There are a number of different ways to wire them, but this diagram is a typical configuration that illustrates how they work:
The travelers go switch to switch - they are the red in that diagram and the black running along the red. So they are both switched hot - one or the other will be hot depending on the position of the switch the feed from the panel comes to, the switch on the left in the diagram.
They are wired so that whenever you throw either switch, if the lights are on they'll turn off, or if they're off they'll turn on. (Study the diagram a minute and that should make sense.) Note that neutrals are not involved in the switching, and in normal house wiring you're dealing with AC so there's really no positive and negative.
When replacing three way switches with occupancy sensor switches things work a little differently. The exact instructions will depend on the switches you select. For example the Lutron Maestro is more or less typical:
Lutron Application Note 435
Lutron makes a switch with an occupancy sensor and a companion switch to replace the two 3-way switches. The wiring is pretty much like regular 3-way switches, getting the common right is the main thing, but there's one big difference: the occupancy sensor switch has a neutral connection.
Since the switch itself is a powered device, it needs a neutral; regular switches do not. Until recently electrical code did not require a neutral in switch boxes. If there is a white wire in one of the boxes that is not connected to the switch, it is probably a neutral.
If you put the sensor switch in the box on the left in the diagram, it's simple - there is an available neutral. If you want to put it in the box at the right, you have a bit of a challenge. Whether it is easy or challenging in your case depends on the specifics of the wiring.
Photos never hurt; clear, well lit photos that make it easy to see all the way to the back of the box are particularly helpful.