Union--> can retrieve the data from multiple tables(of same no of columns and data type) without duplicates.UnionAll-->viceversa to union

you can go through this link.http://www.codeproject.com/Articles/716224/SQL-Server-What-exactly-are-UNION-and-UNION-All

UNION - Only take single entry of row if available in both table in UnionAll is give duplicate entry also

Union will remove the duplicate rows from the result set while Union all does'nt.