Part of Premium Data's US end-of-day service allows you to sort stocks into multiple folders on the basis of their index/sector/sub-sector etc. There's two major classification systems in operation on the US market - Standard & Poors (GICS) and Dow Jones. Premium Data covers both of them.
The constituents of each list are stored in a simple text file and are updated weekly (since stocks change codes quite often and index constituents change too).
Looks like the Bar Chars industry groupings are based around the SIC (Standard Industry Classification) codes.
This classification system, whilst popular in Government circles, is not widely used on the market.
It seems that the various companies involved in the classifications (S&P, Dow Jones etc.) have a vested interest in performing the classifications on these companies so that managed funds, exchange traded funds, indices and derivatives on indices (futures & options) can be made against them.
In the UK, FTSE Group (makers of the FTSE index) seem to dominate the market but look like they also want to expand globally.
The Yahoo sectors as provided by Mokwit above seem to be provided by Hoover's Online, who seem to use their own classification system quite separate from everybody else - I don't know of any funds or indices that track any of the classifications that Hoover's provide, so its value is somewhat limited in my opinion.