Does anyone know what is the difference between L78XX with L78XXC? Is the C meaning capacitor?
L78xxC is the version rated for the commercial operating temperature range, which can only go as low as 0 C. See datasheet.
The non-C version, L78xx, can go as low as -55 C and uses the huge metallic TO-3 can package. You'll probably see it in space/military/industrial applications... if you can find any that is not already obsolete (this is a very old part/package).
From an older ST datasheet: