It means a cookie that is dropped on the computer is valid for 30 days so you
can still earn commissions 30 days later if the user decides to purchase on
their website.

If someone clicks on your affiliate link a cookie is place on there browser that will expire in 30 days. If they leave and within 30 days they decide to purchase from anywhere else the commision still goes to you because they clicked on your link initially. The cookie ties them to your affiliate account for 30 days.

This means if some one visits your link a cookie is placed and for the next 30 days if that person purchases a product the commission payment goes to you.

It's a simple concept, when a visitor of your website clicks on one of your referral links it will generate a cookie in their web browser that will last for a specific amount of time. If this person has the time limited cookie still when they purchase or sign-up for something on the affliate site you'll get the referral credit. However, issues can arise if someone uses a PC cleaner like PCPitStop or System Mechanic. These programs tend to delete internet explorer and firefox cookies when they are ran. Also, some spyware and malware software suites detect these types of cookies as 'tracking cookies' and remove them as well. I'd suggest doing your best to promote the product or service on the web page containing your affiliate link in an attempt to get to do whats called a AN (action now). This is when someone goes directly to your affiliate link to purchase or sign-up right now.

It simply means if the surfer/user buys the product you promoted within 30 days of your referral, you get credit for the sale.   Clickbank sets a 60 day cookie, you should look at them for other products to promote.

Although technically correct answers from everyone an important piece of information has been missed out. Firstly I don't believe that cookies are wiped from people systems at the kind of rates stated here. Pretty much all companies use cookies to give the web site readers a personal experience and a simplified re-login. All of which would disappear if cookies are cleared. Most protection programs will only clear malicious or intrusive cookies like adware/spyware/trojans etc. They do not, as a matter of course, remove your helpful cookies. The main reason an affiliates cookie will be removed is if the visitor goes to another affiliates website after visiting yours (last man in counts!!) as your cookie will be overwritten. This is actually quite likely as someone looking for a product, or deal on a product, is likely to visit a number of sites before making the final purchase. If your site is not the last one visited you will not get the commission. ---This post was edited to remove a commercial link. Read our discussion policies for more Community best practices.

I been a part of affliliate programs in the past and please see below for my understanding on cookie referral period:-   When a user clicks on our banner (everytime) a cookie  (a message given by a web browser to a web server) is positioned to follow the action. And the lifetime of this cookie is termed as referral period. For an instance, if a visitor clicks on your link today and comes back to the site by directly typing in your url within the specific referral time (which is 30 days for you) and completes the action - you would still be getting the commission!   I hope this helps.