'=' in  c is the assignment opperator.whereas '==' is the 
logical equality operator.

= is the assignment operator while == is equality operator

= is assignment operator & == is equality operator in C
= operator is used to assign a value to the variable
== operator is used to compare two variables.

I NEED MORE....

= is used for assigning the value to a variable
for example
int a=10;
if a=5
printf("%d",a);
else
printf("%d",a);

output:5
in this program a value is assigned to 5 and 5 is printed.

whereas
== is used to check the value of the variable
for example
int a=10;
if a==5
printf("%d",++a);
else
printf("%d",a);

output:10

in this value of a is checked with 10 .If it is equal if 
stmt is true and a is incrementd and printed.
If it is not equal a value is printed.