Keywords. It's all about keywords. Resume screeners don't have the time to go through every resume they get with a fine tooth comb.
Staffing managers at Fortune 100 companies report that they get as many as half a million web-based applications each year. Monster.com claims to add 40,000 resumes to its database each week. Since not even the most dedicated recruitment professional can get through that big a load, she (or he) often relies on weighted, google-like searches to identify preliminary job fits.
So when you're applying for a specific job, make sure that the emphasized keywords in your resume are the same as the emphasized keywords used in the job qualification area of a job-posting. If the fit is a stretch, DON'T APPLY, look for a better fit instead. Why? Because when you click SUBMIT, you're not only introducing yourself to the recruiter (and therefore the firm) via your resume, you're also saying something about your judgement and the respect you have for a potential co-worker's time.
The last thing you want on your file is a note that says "IGNORE-tenth submittal this month. This guy doesn't know what he wants or what he's good at." 
After all, if you do the matching well, the recruiter will yell, "It's a hit!" and call the hiring manager to schedule an interview!
