Ganking

Ganking is the act of actively moving around the map in order to kill an enemy Hero in a numbers advantage.

The advantages of Ganking
Ganking is used to gain an overall experience and gold advantage. Successful ganks can significantly slow down an enemy carry's farm, help a teammate recover a difficult lane, and provide a time window to push down a tower or to kill Roshan. To gain an early advantage, active ganking and pushing are essentially mandatory. Additionally, ganking is often the easiest way to kill heroes when the other team has a large advantage in the late game.

Ganking also serves as a counter to enemies trying to push because they must move deep into your territory, where they are often isolated and have little vision. After the enemy pushers are ganked, your team can execute a counterpush to negate the advantage the other team was trying to gain.

Heroes
Most ganking Heroes have the ability to either slow or stun their enemies, making it easy to set up kills for their allies. There are two distinguishable roles who gank frequently:

Gankers
Gankers, as their title suggests, are Heroes that are extremely efficient in ganking, often destroying their opponents in one-on-one combat using the element of surprise. Some gankers depend mostly on ganking for their farm, and must get kills in other lanes to buy their items. Most gankers begin in middle lane and rush to level 6 to gain ability ranks and their ultimate, possibly excepting Alchemist,  Mirana,  Shadow Demon, and some others. They are also great rune carriers, especially with a Bottle.

An example of Ganking with Axe can be found here.

Roamers
Roamers are Heroes who are focused on warding and map control early game, often scouting territory and spending little time in lanes. Since these Heroes do not need to stay in a lane, they often aid in ganking, especially if they are equipped with a powerful disable. Most roamers are capable of ganking even before they reach level 6, possibly excepting Bane,  Jakiro, and some others. Some roamers, such as Zeus,  Lina,  Leshrac,  Spirit Breaker, and  Lion, are powerful gankers and may make it their primary role.

An example of Roaming with Axe can be found here.

Items
There are some items that both item-dependent and item-independent gankers can use to ease their job. Items like Blink Dagger, Force Staff, Phase Boots, and to some extent, Tranquil Boots and Town Portal Scrolls help you enter battle more rapidly and allow you to surprise the opposing hero. Shadow Blade and Smoke of Deceit are also great items, providing invisibility. Smoke of Deceit is especially useful because it is cheap and increases movement speed tremendously. However, the base store's stock is limited to two. Successful gankers rarely return to base, so Arcane Boots, an Empty Bottle, or Clarity Potions are needed to preserve health and mana. Items that provide hit point regeneration are also helpful, especially Urn of Shadows and Magic Wand. Though not necessary, items that provide additional disables, such as Eul's Scepter of Divinity and Rod of Atos are useful in the lategame.

Preventing enemy Ganks
The most efficient way to avoid a gank is turtling, the act of farming away from enemy territory by preventing your creeps from pushing too far by last-hitting enemy creeps at the very right moment and denying your own. In the early game, it is difficult to perform a gank if a Hero is just beside a tower. Not only will the tower deal high damage, but other enemies can easily kill the ganker by using Town Portal Scrolls to reach the tower in time.

Items are useful in preventing the enemy team from ganking. A team should maintain vision of key areas of the map by placing Observer Wards and using Sentry Wards to reveal invisible Heroes. Furthermore, Town Portal Scrolls can be used to assist allies who are ganked on the other side of the map. Carrying TP Scrolls, maintaining vision, and watching the minimap allow a team to quickly gain a numbers advantage over the gankers and kill them.

A common way to protect your team from ganks is telling your teammate when an enemy leaves their lane. In public games, this is done in team chat. For example, calling out "miss 2 bot", "ss 2 bot" lets your team know that two enemy Heroes have left the bottom lane and may be ganking.

Some abilities like Mirana's Leap are well-suited to escaping from incoming ganks. These abilities are often referred to as escape mechanisms. Heroes lacking an escape mechanism like Shadow Fiend and  Silencer are \susceptible to ganks and must be especially careful when enemy Heroes are missing on the minimap. These Heroes can purchase items like Shadow Blade or Force Staff to gain an escape mechanism.

Other Heroes have abilities that help with map control through temporary vision, pseudo-wards, temporary summons, and illusions. Clockwerk's Rocket Flare,  Vengeful Spirit's Wave of Terror, and most of  Zeus's abilities are prime examples of abilities that reveal the map temporarily for an advantage. Venomancer's Plague Ward is also a great anti-gank tool because the wards provide vision and can be spammed. The minions summoned by Lycanthrope's Summon Wolves,  Enchantress' Enchant, and  Nature's Prophet's Nature's Call provide vision as well. Night Stalker with an Aghanim's Scepter has protection against ganking- the Scepter gives him the ability to see through cliffs and trees at night. Slardar's Amplify Damage, Bounty Hunter's Track, and Necronomicon have some anti-gank utility.