UNION
The UNION command is used to select the related information from two tables, which is like a JOIN command. However, when using UNION command, all the selected columns need to be of the same data type. With UNION, only distinct values are selected.
UNION ALL
UNION ALL command is equal to UNION command, except that UNION ALL selects all the values.
The difference between Union and Union all is that Union all will not eliminate duplicate rows, instead it just pulls all the rows from all the tables fitting your query specifics and combines them into a table.
A UNION statement effectively does a SELECT DISTINCT on the results set. If you know that all the records returned are unique from your union, use UNION ALL instead, it gives faster results.
Example
Table 1 : First,Second,Third,Fourth,Fifth
Table 2 : First,Second,Fifth,Sixth
Result Set
UNION: First,Second,Third,Fourth,Fifth,Sixth (This will remove duplicate values)
UNION ALL: First,First,Second,Second,Third,Fourth,Fifth,Fifth,Sixth,Sixth (This will repeat values)