Dispel

A dispel is the term used for a forced removal of status effects. Dispels usually come together with other mechanics and are rarely seen as a primary mechanic of a spell or item.

Mechanics
Dispels remove status effects based on whether the casting unit is an ally or an enemy, and whether the status effect is positive or negative. Applying a dispel on an allied unit only removes negative effects (also known as debuffs). Positive effects (known as buffs) are never removed. Vice versa, applying a dispel on an enemy unit only removes buffs, debuffs are not removed.

There are 3 different variations of dispel: basic dispel, strong dispel and death.

Basic dispels are able to remove most basic stats altering effects like speed or damage bonuses or slows and silences. However, they are unable to remove most hard disables like stuns or Forced Movement.

Spell immunity may also apply a basic dispel, although not all of them do. Their dispel does not differ from that of basic dispels, in fact, they use the exact same mechanic. However, some abilities periodically check for spell immunity and get their effect removed if such is detected. Spells and effects which periodically check for spell immunity are, , , 's disarm, 's disarm, ,  and 's slow.

Strong dispels are capable of removing everything a basic dispel and spell immunity can remove. On top of that, these can also remove many more status effects, including hard disables. Whether or not strong dispels can remove positive effects is unknown, since there is no source of strong dispel which can be applied on enemy units.

Death applies the ultimate dispel on units. It removes almost absolutely everything. Only a few effects are not dispelled by death.

Auras and permanent effects
Generally, all buffs and debuffs placed by auras are not removable with dispels. Their effects only disappear when getting out of range, their source disappears or when no longer meeting their requirements. However, almost every aura's buff/debuff linger for a short time (usually 0.5 seconds). Only a few aura effects end immediately when their source dies or disappears.

Other permanent effects have inconsistent behaviors with dispels. Some of them cannot be removed, while others can. However, since they are permanent effects, they almost immediately re-apply their buffs/debuffs when it gets dispelled.

Death
When a unit dies, most buffs and debuffs on it are removed. However, in cases where a hero's respawn timer is 0, no dispel is applied.

Removable buffs
As mentioned before, it is only possible to remove buffs off of opposing units. Buffs cannot be removed off of allies, regardless of the type of dispel, except by death. Due to this rule, and the fact that there are no strong dispels that can be cast on enemies, there is no way to determine which positive buffs are removable by strong dispels and as such are marked with N/A.

A list of buffs which can or cannot be dispelled can be found here.

Removable debuffs
These are all debuffs placeable by units and items. Same rule applies here, debuffs can only be removed by allies of the affected units.

A list of debuffs which can or cannot be dispelled can be found here.