Worth noting that this can be a regional difference. In the UK, the terms are synonymous, (though CV is much more common), and the CV is much more in line with the US idea of a résumé.
This is wrong. Both terms are used in exactly the same way in the UK but the expectations are different. In the US, a resume is expected. In the UK, a CV is expected.
People who use the terms interchangeably are wrong in either case, but that wrongness may vary. If someone in the UK says resume they might mean CV, whereas if someone in the US says CV they might mean resume.
No matter where you are the rules are the same. A resume is usually one page and is a summary. A CV is usually about 3 pages (this is more variable) and contains a full work history.
Where there is often some confusion about them being the same thing is that CVs can be "tailored" and indeed it's usual to compress large chunks of work history if it's not relevant to a particular application. That doesn't make it a resume however.
I'm less certain about CVs in the US, but I believe they simply leave out what would normally go in a resume, which would is often what the first page of a UK CV looks like.
