It's kind of like opening your yellow pages to "autos" and finding all the auto companies' paid ads. Only instead of paying a onetime fee to be listed for a certain period of time (like the 2008-9 edition), they pay a smaller fee per time their particular link is clicked. Keeping with the phonebook analogy, instead of people skimming over their name and them being charged for it while the person ends up calling a business listed a few lines away, a company doesn't have to pay every time their name appears as a listing.
Although PPC can often be a good source of new client contacts for companies with no understanding or even knowledge of the existence of SEO, PPC can often run up huge tabs for businesses that don't actually have a large budget to spend, or if a monthly budget is set, they may lose out on later opportunities. Say for example "Company A" has a monthly budget in mind which is met in the first few days of the month by people clicking on their link (who may or may not actually be interested in the type of product or services they offer), they've now run over this budget and will either have to spend more money to continue to be listed or will very well miss opportunities to establish new business relationships with potential clients.
One hang up consumers have with PPC is the idea they are being advertised to. They know a company paid for its listing, and will often prefer a site that has been placed high on the organic list because it's presence there implies it has been thoroughly vetted by both the search engine displaying it so high in the list as well as the many consumers who have previously chosen that page (also indicated by the high listing status).
There are plenty of PPC engines, most notably Google, MSN, and Yahoo, as well as Ask.com, Business.com and LookSmart.com. It is important to note, however, that each of these sites is a search engine primarily, and draws much of its revenue from campaigns such as PPC. Some SEO businesses will set up this service for you for a smaller fee than it would take to optimize your page, but they would be in error if they did it before informing you of the huge differentiations between PPC and organic SEO.
Just remember not to get caught up in the hype. There are several ways to promote your business, and the internet is a great way since people spend so much time on it. After all, you're reading this article on the net aren't you?