It's not too difficult - all you have to do is know how to tell a spell and an ability apart.

Devour is an ability that says - Devour N (As this comes into play, you may sacrifice any number of creatures. This creature comes into play with N +1/+1 counters on it for each creature you sacrificed for this ability.)
As you can see, Devour is not a spell on it's own, it's an ability that happens as you successfully play the creature. So the creature spell itself can be countered, but if no one counters the spell and the creature spell resolves, the devour ability cannot be countered by spells that counters spells.
Meanwhile, Unearth says - Unearth "cost" ("cost": Return this card from your graveyard to play. It gains haste. Remove it from the game at end of turn or if it would leave play. Unearth only as a sorcery.)
As you can see Unearth is an activated ability (You can tell from the "Cost : Effect" style of wordings) from a card in the graveyard. Therefore, it cannot be countered by spells that counter spells.
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Rules knowledge:
The term “spell” is used to refer to a card, or a copy of a spell or card, while it’s on the stack.
(What is the stack?
When a spell or ability is played, it is put on the stack.
If anyone wants to play spells or abilities in respond to the first spell on the stack, it goes on top of it, until no more players plays spells and abilities.
Then the spells and abilities resolve in the order from the top. Last spell or ability resolves first, followed by the second last and so on.)
The stack keeps track of the order that spells and/or abilities were added to it. Each time an object is put on the stack, it’s put on top of all objects already there
Then there are abilities.
An ability is text on an object that’s not reminder text or flavor text. The result of following such an instruction is an effect.
There are three general categories of abilities: activated, triggered, and static. Abilities can generate one-shot effects or continuous effects. Some effects are replacement effects or prevention effects.
An ability isn’t a spell and therefore can’t be countered by anything that counters only spells. Abilities can be countered by effects that specifically counter abilities, as well as by the rules (for example, an ability with one or more targets is countered if all its targets become illegal).
This post has been edited by qjwong: Dec 3 2008, 10:43 PM