Initialization is the way to initialize the variable .it occurs at the time of declaration.
Assignments is the way to assign the value to the variable it can be done anywhere only once with the final identifier.

To understand assignment and initialization it is must to understand declaration.
Declaration is nothing just a way of declaring variable.
java variables assign or given values using one of the assignment

we
 cannot assign a value to a constant but we can initialize it .this is 
the big difference  between the initialization and assignment. can be 
done many times while initialization occur only once.

 
int a; //declaration
a=5; //initialization, assignment operators are used to assign value to the variable

float a,b, sum;
a=5;
b=4;
sum=a+b;//a+b is assigning to sum.