A little question about UV

UV means unique visitor, right? Does this mean that a given affiliate program will give you money every time a unique visitor enters your site? What`s the catch?

If you are in a program that is pay-per-impression (PPI) then, yes, you would be paid every time a visitor visits your site and sees the banner, advertisement, etc. Unless you are getting a very large amount of traffic, the pay would be nominal. More commonly, affiliate programs will be cost-per-action (CPA) where you would be paid every time a visitor visits your site and clicks a banner or link and completes the action that is being paid (e.g. email submit, zip submit, membership sign-up, purchase, etc.).