What's the difference between Shimano road groupsets?

Like its main competitors SRAM and Campagnolo, Shimano has a hierarchy of groupsets with levels offering different weights, number of gears and - of course - prices!

It's important to remember that while each groupset name maintains its place in the hierarchy, they are usually updated every few years. So, for instance, when Dura Ace - Shimano's top-of-the-line groupset - was launched way back in 1973 (as Dura Ace 7100), it was 6-speed; it is now 11-speed (Dura Ace 91xx).

I will try to keep this list current, please give me a shout if I mess up!

Weight Brakes Rear sprockets* Chainrings  Low sprocket max teeth** Electronic
Dura Ace R9170 2389 Disc 11 2 30 Y
Dura Ace R9150 2051 Rim 11 2 30 Y
Dura Ace R9120 2355 Disc 11 2 30 N
Dura Ace R9100 2007 Rim 11 2 30 N
Ultegra R8070 2627 Disc 11 2 34 Y
Ultegra R8050 2353 Rim 11 2 34 Y
Ultegra R8020 2512 Disc 11 2 34 N
Ultegra R8000 2266 Rim 11 2 34 N
105 R7025 Disc 11 2 34 N
105 R7000 Rim 11 2 34 N
Tiagra 4700 2585 Rim 10 2 or 3 34 or 32 N
Sora R3000 Rim 9 2 or 3 34 N
Claris R2000 Rim 8 2 or 3 34 N

* the more spockets, the smaller the jump between adjacent gears.
** the more teeth, the easier the bottom gear will be.