How to Use BGP Communities?

Please inform me how to use BGP Communities. I'm not referring to the
3 common ones. But more importantly I see the AS:Communitynumber
syntax but I'm not sure the purpose of these communities.

Are they used to "tag" certain received and sent prefixes, so they may
be later filtered? Please clarify. I have trouble understanding this
topic.

Thanks in advance.

Joe

.