There are instances (purple portals) which you can choose to venture in “normal mode” or “heroic mode”.
Bosses in Heroic mode will drop epic items and tokens which you can use to buy epic items from vendors.

WoW Heroic Mode Keys

Auchenai Key (Everyone in the party needs this key to be able to play this wow heroic instance)
Requires Lower City - Revered
” Unlocks access to Heroic mode for Auchindoun dungeons.”

Sells in: Shattrath City, Lower City

Flamewrought Key (Everyone in the party needs this key to be able to play this wowheroic instance)
Requires Honor Hold - Revered
” Unlocks Heroic Difficulty for Hellfire Citadel dungeons.”

Sells in: Hellfire Peninsula, Honor Hold (Alliance)

Key of Time (Everyone in the party needs this key to be able to play this wow heroic instance)
Requires Keepers of Time - Revered
” Unlocks Heroic Difficulty for Caverns of Time dungeons.”

Sells in: Tanaris, Caverns of Time

Reservoir Key (Everyone in the party needs this key to be able to play this heroic instance)
Requires Cenarion Expedition - Revered
” Unlocks Heroic Difficulty for Coilfang Reservoir dungeons.”

Sells in: Zangarmarsh, Cenarion Refuge

Warpforged Key (Everyone in the party needs this key to be able to play this heroic instance)
Requires The Sha’tar - Revered
” Unlocks Heroic Difficulty for Tempest Keep dungeons.”

Sells in: Shattrath City, Terrace of Light

WoW Other Instance Keys

Shattred Halls Key (only one in the party needs this key to be able to play the instance)
Kill a Blacksmith orc called “Smith Gorlunk” outside The Black Temple
Loot the (”this-begins-a-quest”)item “Primed Key Mold” from him and take:
* Entry into the Citadel (Completes in Honor Hold for Alliance)
* Grand Master Dumphry (Completes in Honor Hold for Alliance)
* Dumphry’s Request (4 Fel Iron Bar, 2 Arcane Dust, 4 Motle of Fire
* Hotter then Hell (kill a Fel Reaver in Hellfire peninsula)

Questreward for the quest “Hotter then Hell” is the Key to Shattred Halls

Shadow Labyrinth Key (only one in the party needs this key to be able to play the instance)
Unlocks the gate to Shadow Labyrinth

Drops from the Last boss in Sethekk Halls

Key to the Arcatraz (only one in the party needs this key to be able to play the instance)
” Unlocks the Arcatraz satellite of Tempest Keep.”
World of aWarcraft Questline: Starts in Netherstorm, Stormspire (I found out that there are some other pre-quests here, starts at a consortium guy in Area 52)
-Triangulation Point one
–Triangulation Point two
—Full Triangle (elite)
—-Special Delivery to Shattrath City
—–Breaking in to Arcatraz (Completes in The Botanica and The Mechanar)

The Master’s Key (Everyone in the raid needs this key to be able to play the instance)
Unlocks the Karazhan Instance
World of Warcraft Questline: Starts just outside Karazhan
-Arcane Disturbances
–Contact from Dalaran
—Khadgar
—-Entry Into Karazhan (Completes in Shadow Labyrinth)
—–The Second and Third Fragments (Completes in The Steamvault and Arcatraz)
——Now, complete Black Morass in Caverns of Time and you’ll get the key

Tempest Keep, Eye of the Storm key
WoW Questline starts in Shadowmoon Valley at a guy named “Oronok Torn-heart” (I found out that there are some other pre-quests here and they start in Wildhammer Stronghold/Shadowmoon Village)
First you do some small quests for him, then he sends you out to find his 3 sons.
After you have found them and done all quests you get a quest to kill a golemagg-lookalike (5 man group)
When you have done that Oronok Torn-heart sends you to Shattrath City and there you can
pick up 3 quests and in all 3 it stands like this:
” Four trials you must pass before you are deemed worthy of the Tempestkey”
” Trial of the Naaru: Mercy (Heroic)” Completes in Shattred Halls,Heroic
” Trial of the Naaru: Strength (Heroic)” Completes in Steamwault andShadow Labyrinth, Heroic
” Trial of the Naaru: Tenacity (Heroic)” Completes in Arcatraz, Heroic

After all this is done, you get the Tempest Key

Caverns of Time, Mount Hyjal questline
First you need to save Thrall in Durnholde keep after that is done you can do Black Morass
In Black Morass you can take a quest from a guy in the beginning of the instance and when
you have killed the last boss in that instance and completed it, you can take the quest:
” The Vials of Eternity (raid)” just outside Black Morass.
Completes in Eye of the Storm(Tempest Keep) and Serpentshrine Cavern(Coilfang Reservoir)

After this is done, you will most likely get the key to Mount Hyal

Serpentshrine Cavern Questline
Halfway through The Slave Pens Heroic (Coilfang Reservoir) there was a guy that gave me the quest: The Cudgel of Kar’Desh and the quest reward is:
” The following Spell will be cast on you: 50000 yd range Instant
Allowes the wearer to enter Serpentshrine Cavern: Lair of lady Vashj”
Quest Objectives: Loot the Earthen Signet from Gruul in Gruuls Lair and the Blazing Signet from Nightbane in Karazhan