An Attribute is similar to a tag, except that you can also assign Values to each Attribute. If you're familiar with Tags and Groups, it's helpful to think of an Attribute as a group of tags. For example, you may want to have a single Attribute called "Employee Level", with values like "Entry Level", "Management", "Executive", etc.
A common use of Attributes is to track lead sources. Attributes (and their values) can be passed through to CRM systems using their lead capture forms.*
*This feature is only available for Enterprise customers. Please contact Sales for more information.