Hydraulic lifters work on oil pressure, car wise very common and pretty trouble free up to high mileage
Hydraulic lifters started with 1200 and remove need for valve clearance adjustment
don't know why they stopped using them on 1800 although a magazine article I read recently on a sport bike that went for it's clearances checking (shims) at 50,000 miles required no adjustment. set em right and forget, materials and technology must be better these days. Maybe that's why they gave up on hydraulic and the cost of course, you pay more money to get the cheapest option these days.
I believe the 1200 and 1800s both use shim and bucket. It's actually a hydraulic type lifter with an ajustable rocker shaft that's eccentrically mounted and adjusted for total lash with shims. The 1000 and 1100 didn't have lifters per se, just adjustable rockers that rode directly on the cam shaft. The 1500 has automotive type hydraulic valve lifters.
Solid lifters do need adjustment now and then but tend to reduce valve float at high rpms. The 1500 was readlined at fairly low rpms and got by just fine with hydraulic valve lifters.