This "Email to Salesforce" allows users to BCC a particular email which Salesforce then takes and links to the record based on email address. This has to be enabled, and then users can create a contact record in Outlook, Gmail, etc. to commonly email to this address (the BCC is just too long to copy and paste each time).
First you need to enable the feature by going to Setup>Administer>Email Administration>Email to Salesforce. Once there, edit that and make sure it is active.
Next, each user has to edit and manage their own settings. Go to <NAME>->Email->My Email to Saleforce. Here you will see the actual email address that we want to use to email to in order to save copies of the email communications (again, these link by the recipient email). There are other options on this page that are worth considering as well such as acceptable senders.
One important item is towards the bottom under the associations. I always recommend that you have "If not matching records are found, create a task....." checked.
If you do that, Saleforce.com will leave those emails that it can't link in your "My Unresolved Items" on your home page.
Finally, create a contact called "BCC SalesForce" or something like that with the email address from this page. That will allow you to quickly connect to that contact either on the CC or the BCC fields as needed.