PPC
Pay-per-click (PPC) is an advertising model where you pay for every time someone clicks on your ads. You typically bid on relevant keywords that when searched for in Google will have your ads displayed. Whether your ads are displayed and the frequency with which they are displayed depends on not just how much you're bidding on the keywords compared to others, but also on the quality of your ad, which is represented by several factors.
There are many factors that affect the success of your PPC ad campaign.
The quality of your ads is determined by several factors, one of which is the click-through rate (CTR), which is the percentage of clicks vs views of your ad, or in other words, the likelihood that your ad will be clicked when shown. Another factor is ad relevance, which is how closely your ad matches the intent of the user's search. Then there is the quality of your landing page which is where the person who clicked on your ad will be sent. It's important that it is relevant and useful to the users who click your ads. It should also be enticing, and targeted content with a specific call-to-action and minimal distractions. Whether your ad is displayed will also depend on how many others are bidding on the same keywords at the same time and how much you are bidding compared to them.
When to use PPC
PPC is a good model to use when your web site is not yet bringing organic traffic, or is a way to create additional traffic to organic results.