Ldom is a full blown domain, where you can install the version of Solaris that you want as long as it supports beeing a ldoms guest.
Ldoms is more similar to Vmware esx server where you have a server that you create virtual hosts on and then install the OS in them.
A zone is created inside the OS, and runs closly connected to the OS, they use the same kernel etc.
More info can be found here:
Bob Netherton's Weblog
Linux in a ldom should work, see here:
The HyperTrap