Segmentation is the act of grouping your contacts by their characteristics or the actions they take. Segmentation lets you personalize your messages, so that each contact gets the information that is most relevant to their needs.
You can segment and refine your contacts in lists, tags, or custom fields. Lists are a broad form of segmentation and usually indicate a contact’s status: active, customer vs. non-customer, newsletter subscriber, etc.
A tag indicates contact attributes that are time sensitive or subject to change; things like product/service interest, event attendee, or new employee.
A custom field is a specific data point within a contact’s information profile. Examples of custom fields are job title, height, twitter handle, age, employee start date, etc.