One way to describe VLANs: they are like a private line circuit, separate from everything else (in this case virtual). All the ports assigned to a given VLAN can only communicate with themselves and nothing else, unless you use a Layer 3 device to provide a means of routing to other subnets or gateways, etc.
I believe the 3Com documents have a glossary of terms, including VLAN. Have you not read the documents for the 3300?
They do appear to be confusing at times. Their examples are not always clear.
....JIM....