Voluntary or Compliance?

There are two 'classes' of carbon credit - compliance or voluntary - both suited to different types of buyer. In reality though, both types can be generated from the same types of project, and what distinguishes them is the certification processes they have been through.

 Compliance credits...

These are mandatory: it's where governments cap the carbon emissions of a particular group of carbon-heavy industries (e.g. oil refiners, paper mills, etc.) or where groups of countries cap their own emissions, by allocating each company or country a set number of carbon units to use up (which will be less than their expected need for those units, forcing them to reduce emissions and allowing them to 'sell'  any leftovers to those who have gone over their limits, or forcing them to buy units if they go over their allocation). This way an overall  target level of  emissions is enforced, without having to specify which individual businesses meet what targets. (See Glossary : 'Cap and Trade'.)

It is also possible to generate compliance credits from outside the group of companies or countries trading allocated credits, and sell them to these businesses or countries. In this case the project will go through a certification process that checks that it meets the standards for the particular compliance regime. 

Voluntary credits...

These are what Forest Carbon deal in and they're on the rise - the carbon neutral HSBC, for instance, set about achieving  their goal by buying Voluntary Carbon Standard credits. Companies who aren't (yet) obliged by law to account for their CO2 emissions often go about 'offsetting' them anyway. Their motivation is corporate pride and a genuine collective concern for the environment.

Good quality voluntary credits will also go through independent verification of their claims (as Forest Carbon's do), and in reality many voluntary credits will be of as good a quality as compliance ones.