Thanks, but those are technical differences, not functional differences. I'm asking about how much damage they do, what they're used for, stuff like that.
Do you mean in rules or in lore, then?
Either or. Both are good.
Well, then:
Pulse technology shows up mostly in Tau-portable infantry weapons, with the battlesuit-mounted versions being equivalent to strapping a bunch of pulse rifles together. They damage in a fashion midway between a lasgun and a bolter, being less explosive than the latter and less focused than the former, although they outrange both. The much larger Pulse Submunitions guns, and to a greater extent the Ordnance Multi-Driver, are closer to Pulse cluster bomb launchers, and are mounted on large battlesuits and used for area denial.
Plasma technology is standard Plasma: armor-piercing heavy weaponry intended to crack heavy infantry and tanks. Tau plasma is less damaging than the Imperial type, both to its target and to its operator, and is also carried exclusively by vehicles.
The Tau actually have two kinds of Ion weapons: the old kind are vehicle-mounted artillery and there's a newer man-portable kind. While the old-style Ion Cannons and Accelerators are used as anti-vehicle weaponry, the new Ion Rifles and Ion Rakers are more like ion machine guns. That said, the new overload function apparently lets anything fire area-effect Ion explosives at the expense of safety, so arguably all Ion weapons are at least partly anti-infantry now.