There are lots of different types of offsetting project, so the project matters as much as the purchase. One carbon credit equals one tonne of emissions: where you are left with a tonne you cannot reduce any further, having done everything you can, buying a credit balances that footprint.
It is important to use the right project. ICROA-approved carbon offset projects, verified or certified to VCS, CDM or the Gold Standard, are what ensure the credits are genuine, wholly additional and credible, and can evidence that the carbon saving actually happened. It is also important that those credits are then officially retired from existence and that the retirement is published on an environmental registry - that is what makes the whole process transparent and lets you fully evidence any claim of carbon neutrality.