Tags and Groups
CiviCRM provides a number of approaches to segment or categorise contacts. Tags and Groups are the most commonly used.
Groups are the building block for selecting contacts for Bulk Mailings. There are two types of Groups, Standard Groups and Smart Groups. The latter is a 'saved search' so it is the characteristics which are saved, and the contacts in the group will change dynamically as contacts gain or lose those characteristics.
An example of a Smart Group would be 'contacts who have donated over $1000 in their lifetime".
To create a Tag, use this link.
To create a standard group, you can either use this link, or you can do a Search and then create a Group from any of those contacts.
To create a Smart Group you need to conduct a Search, and then select ALL the contacts and use the Action > New Smart Group.