Say we have a C/G (C chord with G in the bass). Is this a Slash Chord or Second Inversion of C Major chord? Are slash chords and inversions the same exact thing?
Typically, yes: a slash chord is just an alternative way of notating an inversion.
C/G, as you said, means "a C-major chord with G in the bass," which is exactly the same as a C-major chord in second inversion.
But not all slash chords are understood as inversions. The notion of the "rock dominant," for instance, is a IV chord above V, or F/G in the key of C.