The Native VLAN is an often confused concept, though it needn’t be. This video will explain what the Native VLAN is and how it affects traffic on a wire.
An Access port (or “untagged port” in the non Cisco world) is a switch port which carries traffic for only one VLAN.
A Trunk port (or “tagged port” in the non Cisco world) is a switch port which carries traffic for multiple VLANs.
When frames traverse a Trunk port, a VLAN tag is added to distinguish which frames belong to which VLANs. Access ports do not require a VLAN tag, since all incoming and outgoing frames belong to a single VLAN.
The Native VLAN is simply the one VLAN which traverses a Trunk port without a VLAN tag.