Continuous effects can be generated by either static abilities of permanents on the battlefield, or by spells and abilities as they resolve. Once they start applying, the source of an effect is irrelevant: we handle them all in the same way. However, there's a subtle difference between these two types of effects: the rules to identify the set of objects they apply to are different.
Effects from static abilities
Continuous effects from static abilities continuously re-evaluate their impact on the game. In other words, the set of objects they apply to and what they do to them is never "set in stone", and can be modified by play actions.
Example. I control Glorious Anthem and animate my Treetop Village. The enchantment will see that a new creature has appeared on my side of the board, and start applying to it: the Treetop Village will go straight from being a land to being a 4/4 land creature. It's never 3/3. If I lose control of it because my opponent steals it with Dominate, Glorious Anthem immediately stops boosting it: my opponent will receive a 3/3 creature, and will never control a 4/4, not even for a split second.
If I cast a creature with Glorious Anthem into the battlefield, it will enter the battlefield already boosted. For example, if I cast a Giant Spider with a Garruk's Packleader and a Glorious Anthem on the battlefield, I'll get to draw a card.
These effects begin as soon as their source enters the battlefield. There's no moment, however brief, then their source is into the battlefield but they're still "off". In the same fashion, these effects immediately expire if the permanent they come from leaves the battlefield.
Effects from resolving spells and abilities
Very often, these effect will have a well specified duration. In this case, they always say that something is true for as long as a condition is met. In case this duration never starts, or ends before the effect begins, the effect does nothing. It doesn't start and immediately stop again, and it doesn't last forever. In case no duration is specified, the effect will last for the remainder of the game. ,
Example. If you cast Sower of Temptation targeting your opponent's creature, and respond to its triggered ability casting an Unsummon on the Faerie, the ability will resolve but won't have any effect. It will not switch control at all, so the targeted creature won't even be affected by "summoning sickness" again.
Dominate doesn't specify a duration a duration, which means I'll get to keep the creature until it dies or it's stolen back from me with another spell.
The behavior of these effects change depending on what they do. If they alter the rules of the game, they'll work in the same way as effects from static abilities: they "circle around" and look for stuff to affect according to their text. If they modify characteristics or control of objects, on the other hand, they will enumerate the objects they affect and compute what they do to them only at the time they resolve, and then stick to this for their duration.
Example. If I resolve Glorious Charge (that interestingly has the exact same text as Glorious Anthem, except it's an instant rather than an enchantment) and later animate a Treetop Village, the land creature will not be affected, since was not part of the set of object Glorious Charge looked for at the time it resolved. On the other hand, if I animate it, cast Glorious Charge to pump it, and my opponent steals it with Dominate, it will keep the bonus, since the pump effect is locked unto it.
Let's say I cast Inner Calm, Outer Strength targeting my Gnarled Mass. As it resolves, I count the cards in my hand: they are three, so the Spirit will get +3/+3. If I later cast more spells or draw a bunch of cards, the bonus granted to my creature will not change.
Example. Consider the card Teleportal. Even though its text is only a single sentence, it actually has two separate effects: it gives its target +1/0, and it states that it can't be blocked. If I play it for its overload cost, and then animate an Izzet Keyrune, it doesn't get +1/+0, since it was not a creature when the effect first calculated the set of creatures it would apply to. However, it can't be blocked, because this effect is not changing its characteristics (it doesn't say "Creatures you control gain 'This creature can't blocked.'"), it's changing the rules of the game.
Here's a table that sums up what we have discussed so far:
|Effects||from a static ability of a permanent||from a resolving spell or ability|
|that modify characteristics or control||constantly re-evaluate what they do to what||evaluate their effect only as they resolve|
|that alter the rules of the game||constantly re-evaluate what they do to what||constantly re-evaluate what they do to what|
Interaction of continuous effects
Continuous affects are aplenty in Magic, and since they have a duration, they often interact with one another. Since the final result can be very difficult and ambiguous to evaluate, a comprehensive framework exist to handle such interactions. Given the preminence it has, it was split into its own section.
|One-shot effects||Effects||Replacement effects|