Handicap reduces player character's strength and viability to offset perks/abilities and general balance issues.
Handicap/Perk balance creates a unique build for every character.
Types of handicap:
1.Faction handicap: e.g. a race/species living in water in slower on land. Offsets Faction perks/bonuses.
2.Class handicap: e.g. archers are bad with melee weapons. Offsets class perks/bonuses.
3.Skill handicaps(usually called a skill limitation/immunity): limits on areas or periods where skill can be used. Not related to cooldowns and mana costs. Skill handicap manifests as inability to use skill in certain situations/areas.
e.g. skill can't be used on birds or underwater.
Skills can also have specific handicap rules such as "no items can be equipped during skill use" which change gameplay significantly.

4.Item handicap(related concept is "Cursed items"): using a specific items prohibits some other items from functioning or being equipped. Other variations just doesn't allows using the item itself or disable its functionality, unless the player fits specific handicap rules(such as "no other magic items can be equipped" or "skills which use fire element are prohibited during equip of this item").

5. Quest handicap: a temporary handicap designed to reduce player's power to complete a quest with more challenge.
Restrictions on skills and transporation are common options.

6.Condition handicap: Unlike Curses which only provide negative effect to their victim, condition handicap is a side-effect/restriction/debuff coming from applying a buff/aura on a player.
e.g. within the range of Aura X you can't use potions.

7.Area handicap: Area property prohibits certain skills or items.
e.g. you can't cast fire spells underwater and can't use throwing bombs.