What is the difference between %*c%c and %c as a format specifier to scanf?

In a scanf format string, after the %, the * character is the assignment-suppressing character.

In your example, it eats the first character but does not store it.

For example, with:

char a;
scanf("%c", &a);

If you enter: xyz\n, (\n is the new line character) then x will be stored in object a.

With:

scanf("%*c%c", &a);

If you enter: xyz\n, y will be stored in object a.

C says specifies the * for scanf this way:

(C99, 7.19.6.2p10) Unless assignment suppression was indicated by a *, the result of the conversion is placed in the object pointed to by the first argument following the format argument that has not already received a conversion result.

According to Wikipedia:

An optional asterisk (*) right after the percent symbol denotes that the datum read by this format specifier is not to be stored in a variable. No argument behind the format string should be included for this dropped variable.

It is so you can skip the character matched by that asterisk.