When employers post jobs online these days, it is amazing how many applications we get from job seekers who are clearly over- or under-qualified. Applying online makes it so easy, job seekers obviously think they may as well toss their hats into the ring for any job. Generally, this is a big mistake: it is quite simply a waste of your time and the employer's.
For all intents and purposes, over- or under-qualified = unqualified.
Having said that, if this is the challenge you face with respect to a particular opening, you have one slim chance. If you can convince an employer that your level of relevant experience is commensurate with what they are looking for, they may overlook "chronological discrepancies." Here's what I mean...
If you are overqualified, you can try to position yourself as having limited experience in the exact area I'm looking for -- corresponding to the desired experience level. Let's say I want to hire a web marketer with 4-6 years under their belt, and you have 10 years of general marketing experience, 4-6 of which have been in web marketing. If you can convince me that you have 4-6 years of web-specific experience, and it's what you want to focus on, I may consider you. Of course, you will need to overcome my suspicion that your ambitions (and salary requirements) are beyond what I'm looking for. Note that I may even appreciate the other experience/skills you bring to the table -- but I don't want to pay for them, and I don't want to worry that they mean your interests are off-target.
If you are under-qualified, the challenge is even greater. You need to convince me that you have either accumulated an extraordinary amount of experience in a limited time, or that the peripheral experience you bring to the table (perhaps volunteer work) makes up for the fact that you fall a few years short of what I'm looking for.
Frankly, both are a long-shot. Chances are, employers will take one look at your resume and discard it. My best advice, for circumstances like these, is to network your way to meeting the person who's hiring. They may be more inclined to at least consider the possibility that you are the right candidate.