Unions are compilation of multiple parts in your layout. Union of parts will move them together as a group but you are the one who will be doing the movement. It is very handy for and IC and decupling capacitors to make a Union out of them and move them together.
Cluster can be created and moved manually like Unions. However Clusters are also part of Automatic Cluster building and Cluster Placement. You can auto create Clusters and then Autoplace Clusters. I recommend doing some experimenting to get the feel how 'Build Clusters' and 'Place Clusters' Setups can control its behaviors.
In addition to experimenting, please look into help as it has a lot of good information that will help you get familiar with these routines.
Thanks for your reply.i created union which parts are U2,U5,R3,R5,c10,c12 but after that i need to add extra parts in this union how to add it ?? is it possible to combine two union into one ??
Yes it i possible. Select existing unions and break them. Select components from brooken unions and re-create a bigger union.