Channeling or channeled abilities are abilities which require the caster to remain stationary for the channeling duration, while regularly causing an effect. A time bar will appear and it goes from full (with green) to empty. Generally, channeling spells also have a casting time, although it is zero (hence instant). However, some exceptions with a casting time above zero do exist, as mentioned below.
Additional notes:
- Usually attacks that hit the caster will put the channeling time bar closer to being empty, reducing the ability's duration.
- Some attacks will interrupt the spell-casting completely and the time bar will fill completely with red and say "Interrupted".
- Some abilities, powers, or talents can prevent interruption.
- Mana is consumed before the channeling starts.
- Ordinary mana regeneration stops for 5 seconds after starting to channel (if the ability is channeled for less than 5 sec) or for the duration of channeling (if the ability is channeled for more than 5 sec; see the Five second rule).
- The player can interrupt their own spell at any time during the channeling by either moving, jumping, or pressing the Escape key.
Currently, all channeled spells, with the exception of 
[Arcane Missiles], are calculated as periodic effects (Damage over Time, Heal over Time).
Using bandages is a channeled ability as well.
Channeled Abilities with a Casting Time
Some channeled abilities still have a Casting Time, such as 
[Mind Control]. Their behaviour at the beginning is that of a normally cast spell (see Casting Time), and after finishing to cast, they function like channeled spells (as above). So the mana is consumed after finishing normal casting, but before starting to channel.
Special Properties
Damaging channeling spells have some special properties, which make them favorable over normally cast spells in some situations. Arcane Missiles has a special behaviour, as it technically is not a Damage over Time as any other channeled ability. In the list it is stated where AM does not function like normal channeled abilities.
List of Special Properties
- Crits: Channeled spells are not capable of critical hits. This does not apply to Arcane Missiles.
- DPS while being attacked: If you are attacked while channeling a spell, it will lower your DPS not as much as for a normally cast spell, as your damage is still applied regularly (every tick), regardless of where your channeling bar is. The only case where your DPS is lowered, is when you are interrupted at the end, so that you lose one tick of damage.
- DPM while being attacked: As your mana is consumed before the spell is cast, your DPM is lowered if you are being attacked, as your channeling time and consequently the damage reduce.
- Fixed Damage: Channeled spells regularly cause damage, therefore if you are interrupted while mid-channeling, you will have caused damage proportional to your channeling time. Any normally cast spell will not do any damage until unleashed. For example,
[Fireball] at rank 11 has a 3.5 second casting time. Consider casting [Fireball] versus [Arcane Missiles]. If a mage is interrupted after 3.1 seconds Arcane Missiles would do 192 damage 3 times for 576 damage. Fireball would do 0 damage. However, the Arcane Missiles spell would cost full mana and the Fireball would cost nothing since it was never cast. - Interruption: Channeled spells will not interrupt enemy casters. This applies for either casting or flag-tagging. Although some channeled spells cause the interruption animation (such as
[Hellfire]), they don't actually interrupt. This does not apply to Arcane Missiles. - Line of Sight Check: For a channeled spell, the LoS check is done before the spell is channeled. So if you passed the check and you hit your target, your damage will be applied and your target cannot run out of your LoS anymore (apart from running out of range). This can be very handy against skilled players, as your normally cast spells can easily be "dodged" (by running out of your LoS), whereas for channeled spells you just need to keep pressing the button until the animation appears and the damage will go through. If you find yourself having trouble with such players, it may be more helpful to abstain from using normally cast spells. For Arcane Missiles the LoS check is done for every single missile, however, as soon as your Arcane Missiles are "activated", your character automatically turns into the right direction.
- Resists: Due to channeled spells being binary effects, such spells can only hit or miss; partial resists are not possible. This and the fact that those spells are cast instantly means that a resist will only cost the time of the Universal Cooldown instead of the whole channeling time. Therefore, especially long time channels are only minorly affected by resists. This does not apply to Arcane Missiles.
Conclusion
The conclusion of this is that you have to analyse which is more important between time and mana. If mana is more important, you will want to use the non-channeled spell. If time (thence, health) is more important, you will want to use the channeled spell to do as much damage as you can. Note also that if a channeled spell is fully interrupted (such as with a knockback or a type of stun perhaps) after a couple seconds, you're better off than if you had been casting a non-channeled spell for a couple seconds and then got an interrupt. If you're a mage and have the talent [Improved Arcane Missiles], when you're being attacked melee, there's no doubt at all Arcane Missiles is better than any other one-target spell.
Different classes and situations call for different tactics, however; in melee PvE, a shadow priest who is low on mana but has 
[Power Word: Shield] up will constantly use the mana-efficient, channeled spell 
[Mind Flay], but when the shield falls they will often switch to using a combination of 
[Mind Blast] and their wand to deal damage.