Six Sigma is a system of improvement with the stated aim of achieving zero defects in a given process. The "Sigma" is a statistical term meaning standard deviation. In the case of Six Sigma that measures how far a given process deviates from perfection. Once you have figured out where and how many defects there are, you can systematically set about eliminating them.

Motorola claim over $16b of savings over the last 15 years, that Six Sigma has been spread across their organisation.

Achieving Six Sigma takes a different route from that of the Capability Maturity Model CMMI, which requires an inspection.

Ultimately they can be traced back to the work of Deming and Juran. Motorola claim to be the inventor of the Six Sigma strategy, following the work of Bill Smith.

Originally for manufacturing, its supporters claim that it can be used in anything from Financial Services to Healthcare

The framework focuses organisations on customer requirements, process alignment, analytical rigor, and timely execution. The central core is "DMAIC"

The target in Six Sigma is for 3.4 defects per million opportunities. An "opportunity" is defined as a chance for nonconformance, or not meeting the required specifications

GE list the following as being essential for the implementation of Six Sigma on their website.

The important part of Six Sigma is that of the variance from perfection felt by the customer. Rather than past performance.

Accreditation and training courses are given the rather gung-ho levels of green belt or black belt.

Sources
GE
Motorola University