Are enchantments and artifacts considered spells?
Since you asked for some citations from the official rules:


  111.1. A spell is a card on the stack. As the first step of being cast (see
  rule 601, “Casting Spells”), the card
  becomes a spell and is moved to the
  top of the stack from the zone it was
  in, which is usually its owner’s hand.
  (See rule 405, “Stack.”) A spell
  remains on the stack as a spell until
  it resolves (see rule 608, “Resolving
  Spells and Abilities”), is countered
  (see rule 701.5), or otherwise leaves
  the stack. For more information, see
  section 6, “Spells, Abilities, and
  Effects.”


So basically, pretty much every card in Magic is a "spell" - or rather, becomes one, during the transitional period from being a card in your hand to  being a permanent on the battlefield.  But wait!  Why isn't a land a spell, in that case?  Well, because it has special rules making it an exceptional case:


  305.1. A player who has priority may play a land card from his or her hand
  during a main phase of his or her turn
  when the stack is empty. Playing a
  land is a special action; it doesn’t
  use the stack (see rule 115). Rather,
  the player simply puts the land onto
  the battlefield. Since the land
  doesn’t go on the stack, it is never a
  spell, and players can’t respond to it
  with instants or activated abilities.


The Magic rules are pretty arcane at the best of times (appropriately enough), but I hope that clarifies things for you!

EDIT: Actually, it occurs to me that even more clarification may be necessary.  Enchantments and artifacts aren't spells when they're in play: they're permanents.  They aren't spells when they're in their hand, they're enchantment cards and artifact cards.  But they are spells while on the stack (i.e. in the process of being cast).  I'm sure you already know that "counter target spell" means "stop something on the stack from resolving and becoming a permanent, and send the card to the graveyard instead", but it wasn't 100% explicit from your question!  So, just in case there's any doubt at all, Cancel can stop an artifact or enchantment card from coming into play, but it can't send it to the graveyard from play, or anything wacky like that...