The following are descriptions of the various bleaching processes available in the industry.
Also see the following informative link on a Chlorine Free Processing.

Unbleached
Unbleached paper is either gray or dyed during papermaking. This process involves the least environmental impact. Unbleached paper with post-consumer waste is often non-deinked as well. The ink is visible in the paper in the form of tiny ink dots.

TCF - Totally Chlorine Free
Virgin paper that is unbleached or processed with a sequence that includes no chlorine or chlorine derivatives. It may have been whitened with Hydrogen Peroxide, Ozone or biologically.

ECF - Elemental Chlorine Free
Processed without the most environmentally threatening whitener of all time: chlorine gas (pure chlorine). Chlorine compounds such as chlorine dioxide or sodium hypochlorite (common household bleach) are likely used. These chlorine compounds are much, much more stable than pure chlorine.

PCF - Process(ed) Chlorine Free
Applies to post-consumer recycled content only as it means that no chorine (or compounds) were used “this time around” but there may be traces of chlorine present because of earlier processes. Common bleaching agents are hydrogen peroxide (which, when used, breaks down into water and oxygen), and ozone.

Chlorine Gas
This bleaching process is the most harmful for the environment and is largely responsible for the release of dioxins into the environment. Unfortunately, even today, many mills still use chlorine gas during the bleaching process.