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 win a desperate lane and provide a time window to push down a tower or to kill Roshan. To gain an early advantage, active ganking and pushing is especially mandatory if the enemy team is stronger in the late game.

Ganking also serves as a counter to enemies trying to push because they will be exposing themselves to your territory fast. If ever the enemy team is caught ganked while pushing, your team can execute a counterpush to decimate 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 that do ganking:

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 also depend their farm on ganking, and are usually the ones that grab the kill for them to earn their items. Most of these gankers go for middle lane to rush to level 6 to gain some levels to their powerful abilities with a possible exception to Alchemist,  Mirana,  Shadow Demon, and some others. They are also great carrier for runes, especially if they have a Bottle on them.

Roamers
Roamers are Heroes who are focused on warding and map control early game, often scouting territory and hanging out away from lanes. Since these kind of Heroes are away from the lanes anyway, they often help in ganking especially if they are equipped with a powerful disable. Most roamers are also great at ganking even before they reach level 6 with an exception to Bane,  Jakiro, and some others. Other roamers can also be powerful gankers, to the point of making it their primary role such as Zeus,  Lina,  Leshrac,  Spirit Breaker, and  Lion.

Items
There are some items that both item-dependant and item-independant heroes 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 with a quicker entry to battle. You are less surprising when you are slower, so items that help you in your displacement always helps. Shadow Blade and Smoke of Deceit are also great items, especially Smoke of Deceit because of how it also increases movement speed tremendously and also its cheap price (though, it has a limit of 2 stocks per time). Successful gankers also rarely return to base, so Arcane Boots, Empty Bottle, or some Clarity Potion (for early-game gankers) will help out Heroes for them to be able to constantly gank. Items that help regenerate hit points are also very helpful, especially Urn of Shadows and Magic Wand. Though not necessary, items that disable your opponents longer in combination with your disables such as Eul's Scepter of Divinity and Rod of Atos can help greatly.

Preventing enemy Ganks
Probably the most efficient counter to a gank is turtling, the act of farming away from enemy territory by continually disallowing your creeps from pushing too far by last-hitting enemy creeps at the very right moment while denying your own. Most of the time, it would be difficult to introduce a gank if a Hero is just beside his tower. Not only will the tower deal high damage, but other enemies can possibly countergank faster and easier by TP-ing to the tower.

To prevent the enemy team from ganking successfully, a team should make sure that they have vision of key areas of the map by placing Observer Wards. Furthermore, Town Portal Scrolls can be used to help out allies that are ganked on the other side of the map. Always carrying TP Scrolls and watching the minimap can quickly turn a 2v1 situation into a 2v3 situation, effectively turning the gank into a countergank.

A common way to prevent the enemy team from ganking is telling your team mates when an enemy leaves their lane. In public games, this is done via the ally chat: "miss 2 bot", "ss 2 bot" or "MIA 2 bot" means that two enemy Heroes left the bottom lane and they might be ganking.

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

Other Heroes have abilities that help with map control through temporary reveal, 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 of the ability's tendency to be spammed. Lycanthrope's Summon Wolves, Enchantress' Enchant, and  Nature's Prophet's Nature's Call may also be used as a moving ward as well. A special anti-gank tool is Night Stalker with an Aghanim's Scepter, which gives him the ability to see through cliffs and trees at night. To some extent, Slardar's Amplify Damage,  Bounty Hunter's Track, and Necronomicon has some anti-gank capabilities.