According to the website American Progress carbon offsets can be defined as… 

Carbon offsetting is the act of mitigating greenhouse gas emissions by purchasing credits from projects and programs that reduce emissions from sectors currently unregulated by the cap on emissions. In the current offset market, consumers, for example, may purchase carbon offsets to compensate for greenhouse gas emissions caused by personal air travel or driving, thereby “neutralizing” their carbon emissions.

Carbon offsets are typically measured in tons of CO2-equivalents, or CO2e, and are bought and sold through a number of international brokers, online retailers, and trading platforms. Most companies charge $5 to $20 per ton of CO2e offset.

To get the full Carbon Offsets 101 document visit AmercianProgress.org