Yes, a LUN or NFS export can be used as an ESX datastore.
If you have an NFS export, you don't need to create a LUN, as ESX can talk NFS natively.
An ESX cluster can share datastores, which is the key functionality of ESX clustering.
You can move a VM between datastores, this is known as 'Storage vMotion'. And you can move VMs between any type of data store, iSCSI, FCP or FCoE using VMFS, or NFS. You will need the correct VMware licensing to do this.
A datastore cannot be shared by multiple ESX clusters.