The resume is a summary of what your skills are, what you have done in the past, and gives information about what your objectives are.
Sometimes also referred to as a CV, curriculum vitae, but it is NOT the same. Firstly it depends on the country where somebody asks for a CV: Europeans tend to call it 'CV' (United Kingdom) or 'Lebenslauf' (Germany).
Secondly, it's possible that somebody uses the expression CV just as a synonym for resume.
Strictly spoken, a curriculum vitae sheds much more light on your life, your achievements, etc, than a résumé ever does.
A classic CV is most often asked when somebody applies for highly academic or scientific positions. A curriculum vitae contains for example your affiliations, your publications, dissertations, etc.. This said, it's clear that it will get pretty lengthy, mostly many pages.
In opposite, a resume is a rather brief summary and nowadays tuned for mass reading by HR persons (with no time at their hand).
Even if many job seekers say that it's impossible to pack all their multi-year experience into a 1-page resume... you have not really a choice.
Take following approach before starting writing:
Think of it like you were given a minute in front of an audience and you have to introduce yourself and show your strengths. That's exactly what a modern resume is.
You must catch the audience's (HR person's) interest right in the beginning, your first lines.
A resume should be 1, and maximal 2 pages long.
Some people say it should be a strict tabular (conservative) layout style. Others say, it depends on what type of job you're applying for.
That makes sense because if you are an Art Director or Graphic Designer you could demonstrate some of your skills, but don't stress the design, since it's more about arranging the important information in a way that the information is supported and emphasized by the layout. If you have skills in layouting and have an understanding of reader's perception etc, it's completely ok to experiment with the resume.
If your job is related to the Internet world then it is a good idea to have your resume and or portfolio out in the web. Especially if you are working in the creative industry you should make a website your first choice to express yourself.
Mostly, for your paper resume it is a good idea to stick to a tabular layout, which basically follows the above mentioned structure.
The age factor: if you're a seasoned worker and the job offer didn't explicitly asked for 'seasoned' people, be careful about statements like "experience more than 20 years...". It's better to cover this a little bit and speak about "10+ years experience" because the reader could calculate what age you are and be concerned whether or not you fit in the team that for example has an average age of 27.
In the United States it is not common to include a photo with your resume in order for the HR person to stay neutral? Is that the reason, I guess so. That doesn't make sense because once the applicant comes to an interview they see you anyway. In the Internet era there also exists often the possibility to look up people's photos, so having said this, better don't have your party photos on your facebook site.
When you review your resume try to look at it from the viewpoint of a recruiter, a third person and ask yourself if your resume is easy and fast to read and if the important strengths pop into your eye right away.
A resume has a certain set of sections:
Section: Name and Address and Contact Block
Foreigners should state their status (work permit, resident, ...); for Us Citizens may be the clearance status etc.
Section: Core Skills & Experience
That's a brief summary or a bulleted list with your core skills. A good idea is to state how long you're doing this
With the sections above the HR person or the recruiter has already the base information and knows if you're fitting in general.
The following describes you even more but in details, such as your working history, positions, etc.
Section: Work History
Section: Other information: languages, affiliations (if relevant), membership (if relevant), etc.