Is ∞Σ(n=1) (−1)^n sin(n) an alternating series? Explain?

No. To be an alternating series, the general term is of the form "(-1)^n times a (strictly) positive a_n", or "(-1)^(n+1) times a_n" so it alternates in signs from term to terms. In this case, sin(n) is not always positive or negative so terms (-1)sin(n) do not alternate in signs.