In the UK, how does Child Tax Credit differ from Child Benefit? It appears to me that those who receive Child Tax Credit are fewer than those who receive Child Benefit, as there are more criteria governing Child Tax Credit.
In broad terms, Child Benefit is a universal benefit paid to anyone earning up to £50-60,000 a year* whilst Child Tax credit is primarily means tested (for no/low incomes really).
Both are payable provided the young person is under 20 (not 18) and is in full time non advanced (up to "A level" standard) education or certain types of training.
Child benefit is reduced in stages if someone in the household has a taxable income over £50,000, down to 0 at £60,000.
Child Tax Credit is being phased out and replaced as a part of "Universal Credit".
Another site worth trying is http://www.entitledto.co.uk/ (not a government site) but it will point out any benefits, etc that you may be 'entitled to' from all the available government resources.
Don't be afraid to call HMRC for advice - they're pretty good when they eventaully answer! And Citizens Advice and Money Advice Service are free resources that are always handy.
Google gave me these as good results: http://www.which.co.uk/money/tax/guides/tax-credits/child-tax-credits/ and http://www.which.co.uk/money/tax/guides/child-benefit-explained/child-benefit-and-tax-returns/ from the well-respected Which? site.
I must end with a disclaimer though: everyone's situations are different and the benefits system here in the UK is very complex at times. Always seek proper advice and use this type of forum merely as a pointer to the ball-park area.
Best of luck.