Let's say you have two Cisco switches (A and B). On those two switches you have defined two VLANs - 100 and 200.
You might then choose to have two physical links between those two switches.
Something like: A=B
One link will belong to VLAN 100.
The other link will belong to VLAN 200.
You might configure this like
interface FastEthernet1
switchport mode access vlan 100
interface FastEthernet2
switchport mode access vlan 200
Since each VLAN has its own spanning tree instance, this will work.
If you do the same thing on HP, one of the links will block unless you use multiple spanning tree (MST).
Take a look at
http://www.cisco.com/warp/public/473/147.htmlRegnar