I understand how VLAN's, Trunks and Broadcast Domains work but I have a question. If VLAN's are created to logically separate the switch into separate broadcast domains and these VLAN's are then allowed over a trunk...are they separate broadcast domains or are they all one broadcast domain again?
VLAN's Trunks and Broadcast Domains
So, the use of the trunk doesn't remove any of the VLAN boundaries you've set up on one SW1 when you traverse it. It is to allow multiple VLANs to traverse a single link. This is something that a link (interface) doesn't by nature do. A port can be assigned to one and only one VLAN at a time. So all the ports in the same VLAN can communicate within that VLAN.
To work with the same VLAN on another switch, we could directly connect a port within VLAN 10 on SW1 to a port in VLAN 10 on SW2, those could communicate and the VLAN 10 broadcast domain would span across both switches. The same would go for every VLAN that exists on both switches. This means less ports for devices and more ports to connect the switches. It is inefficient.
A trunk allows for multiple VLANS instead to use a single port to carry traffic from VLAN to VLAN. Allowing the same thing but over a single connection instead. This means all VLANs can access the same VLANs over a single "link". This is more efficient.