Again, for VLAN capable switches, every port has some VLAN assignment. (Although as Jon mentions, L3 switches that also support "routed" ports aren't, logically, within a VLAN, although I understand, under-the-covers, a L3 VLAN switch does actually assign such ports to a VLAN, for internal frame tracking. However, further understand, these under-the-covers VLAN frames cannot be sent directly (like other VLAN frames) to a Cisco trunk port.
BTW, both Rick and/or Jon appear to consider frames from untagged hosts, or non VLAN switches, still in a "VLAN". Well yes and no. Remember a VLAN is a "Virtual Local Area Network", i.e. it emulates what hubs and switches provided before there were VLANs. i.e. LANs. What VLANs allow is multiple instances of what used to be only a single instance (shared Ethernet media) on a hub or switch. Or, as Rick further describes, an instance of a broadcast domain.
Rick also mentions, VLAN ID vs. VLAN tag, which might, indeed, be a point of confusion for you. The VLAN tag is an identifier added to Ethernet frames for VLAN frame identification between devices. A VLAN ID is what, again on a VLAN capable switches, assigns every frame to within the switch. Again, how the switch manages frames (internally) vis-a-vis VLANs, is proprietary.
As Rick also describes, how switches tag VLANs to frames, externally, might be different. ISL, I recall, both predates .Q and was proprietary. In fact, I recall, .Q was defined to allow everyone (else) to use a standard for VLAN tagging (which, also BTW, Cisco was a tad late in adopting as they had ISL).
Just to muddy the waters a bit, if you wanted to "share" (i.e. same broadcast domain) frames between two switches, you might define a port on each switch with the same VLAN ID, e.g. 2, and connect the ports. Or, you might define both switch ports, which you will interconnect, as (Cisco) trunk ports and allows VLAN 2 to use those ports. For the latter, assuming VLAN 2 is not defined as the native VLAN, each switch will add a VLAN tag, to identify frames as belonging to VLAN 2.
But, you might also define one switch's port as VLAN 2, and the other switch's port as VLAN 3, and interconnect them. This will also work, effectively making VLANs 2 and 3 one broadcast domain. It works because the frames, on non-trunk ports (excluding "voice" VLAN assignments) will send receive frames untagged; same as the VLAN 2 <> VLAN 2 case. However, on Cisco switches, with CDP active on the port, it will flag a VLAN mismatch for something like VLAN 2 <> VLAN 3.
