While price is not the defining factor, there is no doubt that it is a key indicator of quality.
List of Groupsets
Below is a list of the three largest manufacturer's groupsets for Road and MTB applications.
Each manufacturer's offering is arranged in descending price/quality.
The number of sprockets of the cassette in each groupset is shown in brackets.
The total number of gears is determined by this number multiplied by the number of chainrings, for example a bike that has a double chainring and a 10-speed cassette has 20 gears, although some of them overlap.
Shimano - 2009
Dura-Ace 7900 (10)
Ultegra 6700 (10)
105 5700 (10)
Campagnolo - 2009
Super Record (11)
Athena (11) - new for 2009.
Older Campagnolo groupsets that were discontinued from 2009 are the lower-end:
Campagnolo also offers 3 triple chainring offerings for steep hill-climbing:
Comp Triple (10)
Race Triple (10)
Champ Triple (9)
SRAM - 2010
In 2006, SRAM released two groupsets for racing bicycles, aimed at competing with Shimano and Campagnolo's offerings. The top SRAM groupsets are called 'Red' and 'Force', being the pro-level and amateur racing level lines, respectively.
It is worth noting that SRAM has trickled down its technology at an accelerated rate to compete, and that each year production groupsets feature several upgrades, some years more than others. In 2010 SRAM have released a new groupset, Apex, which is aimed at a more casual level.
Shimano - 2011
Deore XT (9/10)
Deore LX (9)
SRAM also offer parts under several different marques as they've slowly bought out several smaller, specialist bike part manufacturers. These include:
Brake systems - Avid
Cranksets - Truvativ
Mountain Bike Suspension - Rock Shox
SRAM are phasing out 3 ring front chainrings in their MTB range for a 2 front chainring, 10 sprocket rear cassette setup, commonly found on road bikes.
There are some groupsets which are designed for a specific purpose (track cycling, downhill etc.). Some are just partial groupsets which are intended to be used with other groupsets.
Shimano Dura-Ace Di2 7970 - (10) - electronic shifters and derailleurs intended to be used with regular Dura-Ace 7900 parts (cranks, cassette, chain, bottom bracket etc.)
Shimano Dura-Ace (track) - (1) - Track bikes
Campagnolo Pista - (1) - Track bikes
Campagnolo Time Trial - (N/A) - Time Trial components with bar-end controls, chainrings with oversized toothing and super-light brake levers in composite material.
Shimano Hone - (9) - Enduro and Freeride applications
Shimano Saint - (9) - Downhill and heavy-duty applications
The Shimano Nexus began as a internal hub only and has slowly grown into its own groupset including a higher end internal hub (Alfine, previously Nexus 'Redline'), chain, shifters, cranks, brakes and calipers.
Shimano Alfine - (8) - Internal (planetary) hub gearing, hydraulic disc brakes or mechanical calipers
Shimano Nexus - (8) - Internal (planetary) hub gearing, drum brakes or calipers
This information was copied wholesale from the linked Wikipedia article.