Union:
combine two tables and common values to be retrieved no 
duplicate values

Union All:

All the values to be retrieved when combine two table.
(duplicate values)

Union: It will combine the output of two or more queries &
it will eliminate duplicate  values. 

Union all:It will combine the output of two or more queries
& it will not eliminate duplicate  values.

Union:Only Select Distinct Value
Union All:Select All Values Using Duplicate
All Selected Columns Must Have Same DataType
Example: 
Table 1 : First,Second,Third,Fourth,Fifth
Table 2 : First,Second,Fifth,Sixth

UNION: combine distinct values between two tables
UNION ALL : combine all values and also duplicates values 
between the two tables