Union is a data type that allows you to store different data types in the same memory location, i.e., the program can use any one data type at a time. 

Typedef can be used to give a new name for the extisting types. 

For example, consider the definition given below in your program

typedef int INTEGER;  

Now, you can use 

INTEGER a1, a2;

which will be the same as

int a1,a2;

Typedef can be used to give a nea name for the existing types.

For example , consider the definition given below in your program

Typedef int INTEGER;

Now ,you can use 

INTEGER a1,a2;

which will be the same as 

int a1,a2