PPC advertising can be both a long-term and a short-term strategy. This is because you can control your campaign at will. It all depends on your budget. If you're profiting from your campaign, you can leave it running indefinitely. If you're on a tight budget, you can run the ads for as long as your budget will allow, then pause your campaign at any time (and, of course, you can turn it back on, too, when you're ready).
Pay-per-click advertising is often referred to as 'performance-based' advertising. In other words, you pay for results and not for exposure. To put this in context, think of traditional newspaper classified ads where you are usually charged per word to run you ads. Similarly, you may have come across some websites that will let you run a banner advertisement for X amount of time when you pay up front.
The problem with that form of advertising is that you've no guarantee about results. You could spend hundreds on "pay up front" advertisements, and end up going broke if your ad performs poorly (no one responds, no one clicks on your link, etc...)
The pay-per-click model of advertising, however, gives advertisers like you the advantage by letting you control costs. Instead of being charged for 'publication' or 'ad exposure', your ads are run for free. You don't actually pay anything until someone takes action - i.e., someone clicks one the link to your website - hence the term "pay-per-click".
PPC ads are delivered by the pay-per-click search engines, like:
Those are 3 major player that affiliate focus on in the beginning.
However, there are also a handful of smaller PPC engines (slightly lower volume), which offer high-quality traffic and super-cheap clicks:
You should use a mix of PPC engines in your traffic campaign, especially if you want to get the most traffic possible for your budget. Don't rely on only on PPC engine.
Also , make sure you use both the large and small networks. The best thing to do if you're new to all this is choose just one of the major players, like Google, and one of the smaller players, like 7Search. See what kind of results you get, and then decide if you want to run your campaign on additional engines, or drop one from your campaign to save money.