ESX servers have a whole virtual network within them: guest machines connect to virtual switches and uplinks from these switches to the outside. Although, one term that is not used outside of ESX are "port groups." After reading a bit about them and looking at the various tools the the ESX console provides, I think the best way for a network engineer to understand port groups is to see them as network hubs connected to a single vswitch port. This actually makes sense for multiple reasons:

• All members of a port group share common attributes like a VLAN tag
• All members of a port group can see all of the packets sent by other members of this port group
• A port group is always connected to a single vswitch

Actually, it even makes sense to think of the VLAN tag as being applied to the vswitch port that is connected to the uplink of the virtual hub. Therefore, a vswitch with a portgroup "PG1" that has two members "VG1" and "VG2" would be built using a pswitch and a 3-port hub. The uplink of the hub is connected to a pswitch port. Applying a VLAN tag on that port group then corresponds to configuring the VLAN on the pswitch port.Port groups in ESX are identified by their name, which must be unique within an ESX server. Having the same port group names in different ESX servers, however, makes a lot of sense, especially when moving guests around between them. More on this later.