Here are the key distinctions among the various types of ad networks, as well as useful tips for choosing the right ones for your campaigns.

Quick! Name all the good online ad networks you could use for your next campaign.

Actually, that's a triple-trick question for the following reasons:

• First, because there are far too many good online ad networks to name.
• Second, because the specific website availabilities they represent are constantly shifting, so a top network for a campaign this week might be a second choice network for the same campaign next week.
• Third, because the barriers to entry have sunk so low that new online ad networks are popping up regularly, proliferating faster than desirable website advertising opportunities.

Ten years ago, online ad networks were relatively new and relatively scarce. At the turn of the century, there were only about a dozen online ad networks. Today, there are hundreds (as many as 400, by one count), no one of which can satisfy all the needs of a sophisticated advertiser.

So which networks should you use? The answer is about as complicated as a good response to the question, "Where should you shop?"

Obviously, no single store carries everything you're likely to want or need. And while you might get most of your food and household supplies from a large supermarket, you'll likely continue to patronize a few smaller, more specialized stores that offer certain delicacies and boutique items. What's more, your choice of exactly where to do your spending will vary, day to day and week to week, depending on myriad factors, including price, availability, convenience, your changing tastes and preferences, occasional special needs and many more.

Compared with your personal tastes for everyday food, drink and other items, advertising campaigns often present even more complicated requirements. So it's important to understand where and how to shop the full range of networks for the particular campaign results you seek.

To help you save time and get the most from your online advertising spend, here's a brief rundown of key ad network models.