What is the difference between a CV and a resume?
It depends which country you are in, which is why the answer is confusing and why generic advice often contradicts itself.
In the United States and Canada, the two are genuinely different documents. A resume is a short, targeted summary, typically one or two pages, tailored per application. A CV is a long, comprehensive academic record listing publications, research, funding, teaching and conference presentations, used for academic, scientific and medical positions, and it grows without limit over a career.
In the United Kingdom, Ireland, much of Europe, Australia and New Zealand, CV simply means the job application document. It corresponds to what Americans call a resume, is two pages, and has nothing to do with academia. The word resume is rarely used.
So an American reading a British job advert asking for a CV may prepare something far longer than intended, and a British applicant to an American company may submit two pages where a full academic record was expected. Both errors are common.
The practical resolution is to look at context rather than vocabulary. An academic or research post anywhere expects the comprehensive version. A commercial role anywhere expects the short targeted one, whatever the advert calls it. If genuinely unsure, the length expected is usually implied by the seniority and sector.
A related regional difference is worth knowing. Personal details customary in some European countries, including photographs, dates of birth and marital status, are deliberately omitted in the UK and are actively unwelcome in the US, where they create discrimination risk for the employer.