Groups and Tags are two of the primary ways to organize your data. To decide what to use, you need to understand the difference. In simple terms, a group is a collection of contacts, a set: donors who have contributed $10K or higher. A tag is more a type of contact: a company, a pet owner, a volunteer.
There are also functional differences. For example, only groups can be used for mass mailings and have access control (XYZ group is made available to an admin assistant); only tags can be assigned to contact, activities and/or cases. Both groups and tags can be nested (plumber and carpenter are both children of the contractor)and cross all contact types (as opposed to custom data that can only be applied to a single contact type).
The more you examine your data and what CiviCRM offers for controlling that data, the more you can make informed decisions about which feature to use.
For more information on whether to use groups or tags see http://en.flossmanuals.net/CiviCRM/GroupsAndTags.
There are two types of groups in CiviCRM: group and smart group. A group is a collection of contacts that is available for mass mailings and access control; a smart group is a collection of contacts that is created via a saved search (i.e., all donors in zip code 98502).
Smart groups "remember" the criteria for the group. As new contacts are added that fit the criteria, they are automatically added to the smart group. In addition, smart groups are available for mass mailings, but are not available for access control.
These differences will help you to determine which type of group you will be creating.
To create a regular group and add contacts to the group:
Since this process has a number of redundant steps, you may want to add contacts to a group via the contact record or through the Search results.
Smart groups are memorized searches. When a new contact is added to the database whose record matches the criteria for the search--Viola!--that contact is automatically added to the group. Likewise, when a contact's criteria changes, they are automatically removed from the group. For example, if you have a smart group based on zip codes, when a contact matching that zip code is added, they are automatically added to the group. If they move and their zip code changes, they are automatically removed from the group.
To create a new Smart Group:
To edit the settings for, or disable a group:
GingerFeet STRONGLY recommends that you DO NOT delete items in your database, but disable them. If you delete a group, you do not delete the contacts, but you delete the history that contacts were a part of this group at one time.
Tags in CiviCRM are uesed for high level categorization of your contacts. Tags cross ALL CONTACT TYPES (and can be assigned to activities and cases) as opposed to custom data which can only be assigned to a single contact type.
Tags are unique in that you cannot disable them, you can only delete them. While deleting a tag does not effect contact records, you will of course lose the ability to search based on that criteria.
You can edit and delete tags, as well as add Tag Sets.
Tag sets are tagging structures which allow users to add their own tags on the fly.
While configured identically to regular tags they function quite differently. When you create a new tag set, it creates a new field on the edit pages of the entity's activities or cases as well as in the Tags tab for contacts.
This is a tokenising autocomplete field: as you begin to type, CiviCRM looks for matching tags in this tag set and displays any matches below the field. You can select an existing tag or create a new one by typing the entire tag and pressing the Enter key. The tag will then appear within the field in a box. Clicking on the X will untag the entity (contact, case or activity) that you are editing.
Tags created within a tag set can be viewed and edited from the normal Contacts → Manage Tags (Categories) list. However, tags created within a tag set will only be available within that particular tag set.