General name for the different types of CO2e-certificates: - [CER http://en.wikipedia.org/wiki/Certified_Emission_Reduction]'s from the [CDM http://en.wikipedia.org/wiki/Clean_Development_Mechanism] under the [UNFCCC http://unfccc.int] Emission Trading System. - [VER http://en.wikipedia.org/wiki/Voluntary_Emissions_Reduction]'s from Voluntary projects, that are e.g. too small to pay for all the verification and independent auditing costs required to get to the level of CER's. Often projects start fundraising via VER's and when enough capacity is built, they turn the VER's into CER's. - [AAU http://en.wikipedia.org/wiki/Assigned_amount_units]'s, the criticized "Rights to Emit CO2e/Greenhouse Gasses", or Assigned Amount Units. CO2e-certificates are sometimes referred to as the currency of the "Low Greenhouse Gas Economy", where the USA $, €, ¥, etc. are often referred to as the currencies from the Fossil based Economic-Military-Political complex. These currencies are still largely backed by gold reserves, diamonds, papers with proofs of ownership of large petrol reserves and hence push towards waging wars to get control over these mines, wells and the printing presses of these currencies that control the liquidity and availability of these currencies.
The advantage of CER's is that in order to obtain them, one just has to somehow capture or prevent one ton of CO2e, which can be as simple as planting a tree that absorbs CO2e and turns it into trunk, roots and leaves. The CO2e is available in the air all over the planet, so no need to go wage war over mines or wells, just stick a wind-turbine in the air, plant a tree, do an act of good and since 2005, you're rewarded for it in "money" in the form of a CO2e-certificate.
CO2e-money rewards far more an "act of good" than fossil currencies that don't seem to be as good as taking all factors into account, s.a. environmental factors and certainly not greenhouse gas emissions footprint.