Greenhouse gases trap heat in the atmosphere. They all have different global warming potentials (GWP) over different time frames, the higher the number, the worse the impact. For simplicity of accounting everything is referenced back to carbon dioxide which has a global warming potential of 1. There are over 200 GHGs listed in the IPCC fifth assessment report, a sample are below. Note that in current carbon accounting standards the 100 year GWP is used.