On Contact Field: you can segment contacts for a criterial in a field, for example if field Name Starts with A or field Married is True.
On Contact Activity: You can segment based on the activity of a contact, whether or not you opened an email, whether or not you clicked on a specific link in a campaign, whether or not you sent an email or if an email was delivered or not.
On Contact Tag: you can segment if your contact has or not a tag.