You run an ad spending $100.00. How could you possibly know how many people bought your product? If you don't know your ROI on that particular campaign, how will you spend another $100.00 on it?

Simple is You have to track your advertising campaign. This is done by -

By keeping specific URL in that specific campaign
Ex: http://www.yourdomain.com/ezinead.html
By giving '?source=campaign1' tag to your URL
Ex: http://www.yourdomain.com/product.html?source=ezinead
By using a Click tracking script
http://www.scripts4webmasters.com/clicktracking/index.shtml
Click tracking script is a cgi script that gives you information about a click. The click may be on your website, emails, ezines, banners or anything.

If the click tracking script tells you the number of clicks it is not going to be useful to you. Because you can't differentiate the people buying through your campaign links or your regular links.

So what a Click tracking script should tell you?

1. Tell the number of clicks to tell the click thru ratio:

By this you can optimize your ads. If a specified ad get low click thrus you can change the optimize it.

2. Tell the ip address of the visitor:

By this you can know which ad bringing you the sales. If a particular ip clicking on a specific link buys your product you can know that ad campaign's ROI.

3. Tells the geographical distribution of the clicks:

This is useful when you want to know how many clicks coming from different countries. Like if you see 'jp' with ip address, you can know the visitor from Japan. But this is not possible in all cases.

4. Tell the date and time of clicks:

Some online enterpreneurs are more interested in sending promotional emails in the weekends. But I guess it is subject to their personal experiences. If you know that particular campaign gets lot of clicks in certain days, you can concentrate showing ads at that particular time.

5. Referers:

You can know that which link exchange program or banner ad on web sites bring you the most clicks.

6. Clicks by time and date:

You are able to get the results by time and date. Like your click tracking script must provide daily stats seperately. This is useful to see which day you sales page gets most clicks.