There are two types of search engine marketing campaigns: organic and pay-per-click. Organic (or natural results) is the result of search engine optimization and "traditional" SEM campaigns, while pay-per-click results are advertisements: you place a bid and depending on certain metrics (competition, cost, etc.), your ad will show up and you'll pay every time someone clicks on the listing.
While it's often best to do both together, you might want to opt for one or the other. And while many choose to go with organic SEM, some choose to go exclusively with pay-per-click advertising through outlets like Google AdWords, Microsoft Advertising, Facebook ads, etc. Here are the pros and cons of using just PPC:
Pro: You are only charged for the clicks you get
You won't pay a fee each time your ad shows up (also called "impressions"). You'll only spend when your ad is clicked on.
Con: You can burn through your budget quickly
Depending on the competitiveness of your keyword phrases, you could spend quite a bit to get your ad to show up.
Pro: Location targeting
PPC marketing is the easiest way to target your advertising to certain localities. All it takes is the click of a button.
Con: Fraudulent clicks
Fraudulent clicking is an unethical practice done by competitors and unscrupulous PPC agencies. Since you pay for every click, some may try to kill your budget with repeated clicks in order to get your ad(s) delisted.
Pro: Quick results
Organic SEM efforts can take months to materialize, while PPC efforts are immediate. You'll begin to see traffic almost instantaneously.
Con: Can be fleeting
Once your budget is spent, your ads will no longer appear. And when that happens, traffic to your site can drop off quickly.