In the UK you can no longer make a new claim for tax credits because it has been replaced with Universal Credit (unless very limited circumstances apply). The rest of this article is only relevant if you have an existing Tax Credit claim or if you are in receipt of a Severe Disability Premium.
There are two tax credits, Child Tax Credit (CTC) and Working Tax Credit (WTC).
CTC is a means-tested payment for people with children. It depends on income and not everyone qualifies, but it is worth checking your entitlement. WTC is designed to top up a working household’s income and can give some help with childcare costs. It is also means-tested and usually only paid to families on low incomes. WTC and CTC are both administered by the Tax Credit Office, part of the Revenue.
You may qualify for just CTC, just WTC, both together, or neither, but both use the same form and an application for one is an application for both.
Single people must claim on their own whereas couples who live together must claim as a couple, with both their incomes taken into account.
Your Tax Credit award is worked out by calculating a maximum possible award for your circumstances, made up of all the elements (see How tax credits are worked out) you are eligible for. Your income is then worked out and the maximum is reduced to give the amount you are entitled to receive.
There is lots more information about tax credits on the GOV.UK website.