This depends where in the world you are and what sort of job you are applying for.
I understand that the "one page resume" is standard in the US, but in the UK or Australia for instance, two pages would be more often recommended, while in countries like Germany, or sectors such as U.S. academia, a full career history would be expected. (Thanks tehnyit & Jeff O)
Where a full CV isn't expected however, think of your resume or CV as being a summary of your full Curriculum vitae, tailored to each job application. Include the skills and experience which are relevant to the job you are applying for and omit anything which isn't relevant.
If you are writing a CV to go on a careers search website, you have to think differently again. Include things which you want to work with now, but omit things which you don't want to work with. For instance, if you have 3 years of pick experience (an unusual skill these days), but don't want to go back to that time in your career again, leave your pick experience off your on-line CV.
Mostly these on-line CVs are used to filter candidates by keywords, so if you don't want to be bugged by recruiters contacting you about jobs you aren't interested in, make sure you don't include those skills.
Another aspect, as Sam Woods suggests, is the seniority of the position you're applying for. Even in the UK, I wouldn't expect an application for a summer job (intern) position to provide as long a CV as someone with 20 years experience, applying for a senior position.
Ultimately, your resume/CV is your chance to sell yourself. Don't say that you have great communications skills, demonstrate it with your CV. I once interviewed a contractor with a CV which was approximately 16 page long. It appeared to list every project he'd ever worked on and was double spaced, presumably to make it easier for me to make notes. 95% of those projects were wholly irrelevant to me - he could have trivially condensed his CV to two or pages, which would have at least got him an interview.
Finally, keep a full CV up to date for reference, if you have all of the information you need to hand, it makes it much easier to copy and paste it to generate those highly effective tailored CVs.