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Creatures are considered spells in Magic: The Gathering, as long as they're being cast and are currently on the stack. Any other time a Creature exists in the game, it is just a Creature card or a permanent.
A good rule of thumb is this: if it has a casting cost in the upper right-hand corner of the card, no matter what it is (even 
), it's a spell when it's on the stack. All spells and abilities use the stack to resolve.
Creature spells may be cast at any time during one of your Main Phases when you have priority. Once the spell has resolved, the Creature becomes a permanent.
Official Rules on Creature Spells
The Comprehensive Rules state that a "player who has priority may cast a Creature card from their Hand during a main phase of their turn when the stack is empty. Casting a Creature as a spell uses the stack."
In simple words, a Creature is a spell when you're casting it and when it is on the stack. While it's on the stack, other players can respond.
You can cast a Creature:
- When nothing is currently waiting to resolve
- If it is currently the Main Phase of your turn
- As an Instant if it has Flash, like Cuboid Colony
- As an Instant if you have another card that allows you to bypass normal timing rules. For example, High Fae Trickster lets you cast all spells as though they have Flash.
One thing that is important to remember is that abilities like Flash only change when you're allowed to cast the Creature. It is still a spell that will go on the stack, and it can still be countered by an opponent.
Why does the rule say "as a spell?"
A Creature card is only a spell while it's on the stack. Otherwise, it is considered a Creature card or "a permanent" based on where it is.
Creatures in your Library, Hand, or Graveyard are Creature cards.
Creatures on the Battlefield are permanents.
As soon as you pay the cost and move the card from your Hand to the stack (where spells go before they resolve), it becomes a spell and can be interacted with using cards like Counterspell. Once it resolves, it stops being a spell and becomes a permanent.
Exceptions
Sometimes we don't "cast" Creatures. A card like Sneak Attack lets us get around casting the Creature and puts it directly on the Battlefield. That avoids the stack, so this card never becomes a spell.
We should also probably mention Dryad Arbor, which is both a Land and a Creature. However, it says "Dryad Arbor isn't a spell, it's affected by summoning sickness, and it has "[T}: Add {{}
{}}..."
Because it's not a spell, it can't be cast, nor can it be countered.
Final Answer
Creatures are only considered spells on the stack. In most cases, you are only allowed to cast a Creature in one of your Main Phases. They can be countered while they're on the stack and they become permanents on the Battlefield.
Once a Creature is on the Battlefield, it can be destroyed, attacked, Exiled, or otherwise interacted with using Artifacts, Enchantments, and other cards by you and your opponents.
