Everytime somebody clicks on your ad you pay a fee to the provider. This can range from 0.10 pence to anything like £15.00 for the very high competitive keywords. You can start a PPC campaign using any of the major search engines but i would advise rushing in and doing this as you can spend all of your budget very quickly if you do not know what your doing. As an alternative you can use a third party company that will do everything for you for 1 set monthly fee. See the link below for 1 such company but feel free to shop around if you like. Google search "web touch solutions" If you need any further info just ask.
It's something that requires a lot of keyword research before attempting to avoid unnecessary losses. Mix up your marketing with the big three (as I like to call them): Paid traffic, article marketing, PPC (once thoroughly researched)...Good luck!