A inteface logical units is simiilar to vlans used by other vendors?
Somebody explain about.
Right. So a logical unit is required for transit interface.
Its not trasit interface, its part of local physical interface upon which you create logical interaface - that is the unit.
In the Junos OS implementation, the term logical interfaces generally refers to interfaces you configure by including the unit statement at the [edit interfaces interface-name] hierarchy level. Logical interfaces have the .logical descriptor at the end of the interface name, as in ge-0/0/0.1 where the logical unit number is 1. (or say simply an "sub-interface" for other vendor comparasion)