When you enter your EIN (Employer Identification Number) or SSN (Social Security Number) at websites that are not secured by 128bit SSL (Secure Socket Layer), you are exposing your information to the world in clear text.

When you type in information into a form on a website, and you click submit, the data has to travel from your computer to the server where the affiliate program webpage is hosted. If the affiliate program webpage is secured with SSL, your information is encrypted before it is sent so that your data is secure along the way.

Once it reaches the server, it is up to the owners of the website to make sure your information stays secure within their databases.

How can I tell if a website is secure?

The first thing to do is look at the address bar - does it say https? This means at least part of the website is secure.
 

Internet Explorer 7:

Firefox:

Next, look for the lock symbol to the right of the address bar (IE7, Firefox), or bottom of the browser (IE6). This means that the whole page your are looking at is secure and that everything is in order with the SSL certificate.

Internet Explorer 7:

Firefox:

If the address bar says https, but there is no lock, most likely there is some element on the page such as an image or link that is not being secured by SSL. In this case you should get a popup warning you that everything is not secured.

Internet Explorer 7:

Firefox:

Sometimes there will be something wrong with the SSL certificate that will cause a warning such as:
1) The certificate has expired
2) The site doesn't match the name on the certificate

Should you trust a site that has the https but not the lock?

That one is up to you. It never hurts to be safe or picky, however just because there is no lock it doesn't mean that your data is not being encrypted. If there is an error with the certificate, then it's probably best to not enter any sensitive information (an error is highlighted by internet explorer 7 in the address bar). Make sure to contact the owner of the site if anything is wrong with the site. If they are a legitimate business / website, they should fix the problems.