Resumes keep it concise, CVs focus on details
Resumes essentially give potential employers a glance at your skills and past work experience. They should be one-page long (two at the most), brief in wording, and tailored to the position you're applying for. You should highlight the skills that you think make you a great candidate. Resumes don’t have to include every position you’ve held, or list your experience chronologically. CVs, however, are like records of your entire work and volunteer history in chronological order. These give potential employers an exact outline of what you’ve done and the skills you've acquired in that time.
CVs have additional layers of information
CVs can (and usually do) include things like your research interests, teaching experience, publications, and honours or other awards. Resumes do not, and are primarily used to showcase your skills and suitability for a job.
CVs are academic in nature, resumes are more casual
CVs outline your academic credentials and academic/professional accomplishments. Typically, they’re used when you apply to professional or graduate programs, or to work in laboratories. This is where you’d brag about your GPA, all the volunteer work you do, and any awards, distinctions, and honours you’ve received. Resumes don’t really touch on academics, besides listing any diplomas or degrees which qualify you for the job you’re trying to land.
Both need to include your contact information
Both CVs and resumes need to have your full name, email, phone number, and address. They can also include info like your LinkedIn or online portfolio. Make sure you update the information on both your resume and CV frequently, until you no longer really need them.
When in doubt, send both
In my experience, I typically send both a resume and CV, depending on the position. When I applied to labs for volunteer positions, I submitted both my CV and an updated resume which was tailored to the lab. When applying for honours supervisors, however, I contacted potential supervisors directly and asked what documents they would like. Most wanted a CV, but some were more than happy with a resume. When it comes down to it, you can ask the person in charge of hiring what they’d like, or be like me and provide both.