From Diablo Wiki
Spin2Win (or S2W) is the nickname for a cookie cutter Barbarian build. It's the most powerful and popular build in Diablo 3 through patch v1.07, made famous by Alkaizer, the first ever Paragon 100 character.
The build varies in the details depending on which support skills and passives a player needs to cover equipment deficiencies, but it mainly revolves around the Whirlwind skill which is paired with Sprint: Run Like the Wind. Sprint can be used while Whirlwinding, enabling the Barbarian to hit with all the WW damage and Sprint at the same time. The tornadoes emitted by Run Like the Wind deal heavy damage, but also create a steady series of procs which trigger Fury gain thanks to the Battle Rage: Into the Fray buff.
Thus Sprint: Run Like the Wind is the real key to the entire build, as that buff allows the Barbarian to move very quickly, deal constant damage (even to targets left well behind and off the screen), and simultaneously regain enough Fury to pay for the skills he's using.
The "Spin" refers to the Whirlwind and the tornadoes from Sprint: TLtW. The "win" refers to how powerful and fast this build is, enabling Barbarian who use it to level up very quickly while gaining great item and gold rewards.
 Example Spin2Win
A fairly typical example of the build can be seen below:
The LMB skill varies by personal preference, as do the passives, mostly depending on the quality of gear. Players without top quality gear (or who are playing on higher MP levels) need to use the passives to improve their Fury generation, Life Steal, resistances, defense, etc. Characters in better gear tend to focus more on increasing their DPS.
 An Overpowered Build?
Most players feel this build is overpowered, as it has no real weaknesses and is not very expensive to equip or complicated to operate. With it Barbarians can move very quickly, kill rapidly, overcome resource issues, and avoid most enemy attacks. It's a powerful build for farming, but can also be used for high MP grinding, such as key runs, with high end equipment.