This tutorial understands the difference between tagged and untagged vlan.
Take a scenario where a switch port is a member of untagged vlan 2 and tagged vlan 3 and vlan 4. The generic design of the switch states that a port can be a member of only 1 untagged vlan and multiple tagged vlan. Assume that the switch port is connected to a hub, on which there are three PC‟s. PC1 is a member of vlan 2, PC2 is a member of vlan 3 and PC3 is a member of vlan 4. When the data from the PC‟s (ethernet frames) reaches the switch port, the switch would consider that all the frames are members of vlan 2, since the port is a member of untagged vlan
So how can we differentiate. The network cards on PC2 and PC3 should be configured with 802.1q tagged frames with the appropriate vlan numbers as 3 & 4. When this is configured, the data (frames) send by PC2 and PC3 would be 802.1q encapsulated with the appropriate vlan numbers. This is known as tagged frames. When the tagged frames reach the port on the switch, the differentiation can be made by looking into the value ( 3 or 4) inside the frame. Earlier since the frames were not tagged, the differentiation was not possible making the switch believe that all traffic belonged to untagged vlan 2.