Copies of permanents
Some permanents enter the battlefield as a copy of something else already there, either as themselves (Clone for example) or by an effect (usually a spell or ability) that creates a token copying something.
The copy is of the printed values of the copied permanent, plus any copy effects that have been applied to it. It won’t copy any other effects that have changed type, color, power, toughness, counters, and so on. If it’s a copy of a token, it copies the original characteristics of that token as defined by the effect that put it onto the battlefield.

* Altered Ego copies exactly what was printed on the original creature (unless that creature is copying something else or is a token; see below). It doesn’t copy whether that creature is tapped or untapped, whether it has any counters on it or any Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.

* If the chosen creature has {X} in its mana cost, that X is considered to be 0. The value of X in Altered Ego’s last ability will be whatever value was chosen for X while casting Altered Ego.

* If the chosen creature is copying something else (for example, if the chosen creature is another Altered Ego), then your Altered Ego enters the battlefield as whatever the chosen creature copied.

* If the chosen creature is a token, Altered Ego copies the original characteristics of that token as stated by the effect that put the token onto the battlefield. Altered Ego isn’t a token.

* Any enters-the-battlefield abilities of the copied creature will trigger when Altered Ego enters the battlefield. Any “as [this creature] enters the battlefield” or “[this creature] enters the battlefield with” abilities of the chosen creature will also work.

* If Altered Ego somehow enters the battlefield at the same time as another creature, Altered Ego can’t become a copy of that creature. You may only choose a creature that’s already on the battlefield.

* You can choose not to copy anything. In that case, Altered Ego enters the battlefield as a 0/0 creature, and is probably put into the graveyard immediately. It won’t have +1/+1 counters placed on it by its ability.

Here is another similar example, this time of a spell that puts copies into the battlefield:

* Second Harvest copies the original characteristics of each token as stated by the effect that put the token onto the battlefield. It doesn’t copy whether that token is tapped or untapped, whether it has any counters on it or any Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.

* Second Harvest copies all tokens, not only creature tokens. Notably, this includes Clue tokens.

As opposed to entering the battlefield, some copy effects happen when a permanent that is already on the battlefield, copies something else. When that permanent becomes a copy, it’s neither entering nor leaving the battlefield. Any enters-the-battlefield or leaves-the-battlefield abilities won’t trigger. Also, things that affected that permanent before will likely still affect it.

* If applicable, creatures you control copy the printed values of the revealed creature card. They will retain any counters they had on them. Any Auras and/or Equipment that were attached with them will remain so.

* Any noncopy effects that affected a creature you control, including ones that modified its power and/or toughness, will continue to apply.

* If a creature you control becomes a copy of the revealed creature card, it’s neither entering nor leaving the battlefield. Any enters-the-battlefield or leaves-the-battlefield abilities won’t trigger.

Copies of spells and abilities
When the spell or ability that copies a spell resolves, it creates a copy of the spell. That copy is created on the stack, so it’s not “cast”. Abilities that trigger when a player casts a spell won’t trigger. After players get a chance to cast spells and activate abilities, the copy will resolve like a normal spell.
The copy will be identical to the original spell, including modes that were chosen for a modal spell, X whose value was determined as the original was cast, and the same targets as the spell it’s copying.
However, depending on the spell or ability that created the copy, that player may be allowed to change any number of the targets, including all of them or none of them. If, for one of the targets, that player can’t choose a new legal target, then it remains unchanged (even if the current target is illegal).

You can’t choose to pay any additional costs for the copy. However, effects based on any additional costs that were paid for the original spell are copied as though those same costs were paid for the copy too.

* Geistblast’s ability can copy any instant or sorcery spell, not just one with targets.

* When Geistblast’s ability resolves, it creates a copy of the instant or sorcery spell. The copy is created on the stack, so it’s not “cast.” Abilities that trigger when a player casts a spell won’t trigger. The copy will then resolve like a normal spell, before the original spell resolves but after players get a chance to cast spells and activate abilities.

* The copy will have the same targets as the spell it’s copying unless you choose new ones. You may change any number of the targets, including all of them or none of them. If, for one of the targets, you can’t choose a new legal target, then it remains unchanged (even if the current target is illegal).

* If the spell being copied is modal (that is, it says “Choose one —” or the like), the copy will have the same mode. You can’t choose a different one.

* If the spell being copied has an X whose value was determined as it was cast (like Avacyn’s Judgment’s madness cost has), the copy will have the same value of X.

* If the spell has damage divided as it was cast (also like Avacyn’s Judgment), the division can’t be changed (although the targets receiving that damage still can).

* If an alternative cost was paid for the spell, such as its madness cost or surge cost, the same alternative cost is considered to have been paid for the copy.

Keep in mind some spells check what happened as they were cast to see how they should resolve. Since the copies were never cast, things that happened for the original spell might not apply for the copy.

* If a spell with a converge ability is copied, no mana was spent to cast the copy, so the number of colors of mana spent to cast the spell will be zero. The number of colors spent to cast the original spell is not copied.

A copy of a spell or ability is controlled by the player under whose control it was put on the stack. If the copy says that it affects “you”, it affects the controller of the copy, not the controller of the original spell. Similarly, if the copy says that it affects an “opponent”, it affects an opponent of the copy’s controller, not an opponent of the original spell’s controller.

To conclude this article let’s talk a bit about tokens, and tie things up with copying.

Tokens
* The characteristics of a token are stated by the effect that put the token onto the battlefield. Those will notably include: Name, color, and if it’s a creature – creature type, power and toughness.
* Unless a token is a copy, or unless specified by the effect that created it, a token’s name is the same as its subtypes at the time it was created.
* A token has a converted mana cost of 0 unless it’s a copy of something else, in which case it has the mana cost of whatever it’s copying.

* The token is named “Eldrazi Scion” and has a CMC of 0.

* A token that leaves the battlefield will cease to exist.