Cross ref. emails from small list with a larger list?

I have a large list of emails from costumers that signed up for my newsletter, and I also have a list of emails from customers that bought our product. I've been reading some tutorials, but I can't find an easy way to check the list of ppl who bought the product with the list of people that signed up for the news letter.

When they sign up for the newsletter the keyword search phrase they used to find us is stored. If I can take the list of emails from people who purchased the product and find them in the newsletter list I will get the keywords that led to a sale. Can someone help me how do this?

Thanks a bunch

Thank you, I've looked it up and it was exactly what I was looking for.