When you initially capture a lead in Marketo, you might only have a few key pieces of information about the lead. For example, you might know the lead's first name, last name, and email address, or maybe you have the lead's first name, email address, and company name. This is a great first start, but you still need more information to initiate an effective marketing effort, such as a Marketo Program, to help develop, nurture, and ultimately qualify the lead.
Data.com is a collaborative database that contains business contact information. If you have an active Data.com account, you can enrich your existing leads in Marketo with information from Data.com. With Marketo and Data.com, you can capture better, cleaner, and more accurate data for your leads.
Please review the documentation for details: http://community.marketo.com/MarketoTutorial?id=kA250000000KyuPCAS