Actually tagging is way for you to transport data from multiple VLANs over a single connection, thereby utilizing less ports from each switch. Instead of using one port from each switch for each VLAN, you can use one port from each switch for almost ANY numbers of VLANs. (the limit has to do with which manufacturer of switch you use)

Hope that made sense.

Buy CCNA companion 1 if you need to know, or for that matter any other beginner's networking book.


Peace