char *str = "This is string"; - This will keep the string in text segment as a read only data and it will store the address in the local pointer variable str.
str[0] = 'a'; //This will leads to crash, because strings are in read only segment.
printf("%d",sizeof(str)); //This will print 4(in 32bit m/c) or 8(in 64 bit m/c)
char str2[20] = "This is string"; - This will keep the string as character array in local stack.
str2[0] = 'a'; //This will change the first character to a
printf("%d",sizeof(str2)); //This will print 20
