This is the summary page for the carbon offsets tag, but there are also lists of other carbon offsets wikipages, as well as of forum posts and files.
A carbon offset is an activity that soaks up carbon emissions made elsewhere. Because carbon dioxide is spread throughout the global atmosphere, the place where emissions are released or absorbed is theoretically unimportant. It is typically used to remove emissions from activities deemed essential and for which there is no feasible alternative energy source than fossil fuels (e.g. flights).
Carbon offsets have been controversial because they may absorb over much longer timescales than the original emissions, cause no extra emissions cuts than would have occurred anyway (i.e. fail to show additionality), or fail to secure carbon properly in the future.