As stated in the topic, proper Warding can make a lot of difference for your chances of winning a game. I am no expert myself, which is why I'm hoping others will contribute to this discussion with their own insight. This is very much a work in progress. Eventually, I hope to distill the insight gleaned from this thread into a proper guide for both new and experienced players to help warding become more widespread.
1: The basics of a ward
Wards are consumables you can buy. they are stackable, allowing you to carry up to 3 wards of the same type in a single inventory slot. When used, wards allow you to place a static friendly unit on the map. This unit persists for several minutes. (4.5 for sight, 3.5 for vision). Once placed, the ward will turn invisible after a few seconds. After this, the ward is invulnerable, unless attacked by an opposing champion who is able to see invisible units. Wards come in two flavours: Sight Wards provide you with regular Line of Sight when placed in a large-ish area. Vision wards are more expensive. They provide the same basic functionality of a Sight Ward, but in addition, they will reveal nearby stealthed units. This includes: Other wards, Jack-in-the-Box'es, Mushrooms, Evelynn, Shaco, Twitch, Teemo. Wards LoS functions identically to normal champions. That is: A ward cannot see what happens in a bush, unless the ward is placed inside the bush itself.
1b: Oracles Elixir and towers
Oracles Elixir allows opponents to see your wards. If you have a stealther on your team, the opposition is very likely to have someone with an Oracles potion during the mid and late game stages. Once you notice this, be aware that your wards are likely to last markedly shorter as the opponent may go ward-hunting. Likewise, placing a ward close to an enemy tower will allow the other team to see it. (Although not for long, as the tower will shoot it dead as soon as you've no minions nearby. )
2: The uses of a ward
Wards have 6 distinct uses in the game. The first two are fairly self-explanatory.
1: Early warning. If the opposing team has champions you cannot see on the minimap, there is a fairly big chance they are hiding in a bush or jungle somewhere, waiting to gank one of you. Wards can alert you to the movement of enemy champions, so you know which lanes have to be careful and which lanes can afford to push.
2: Revealing targets of oppertunity. Conversely, warding places an enemy may be vulnerable (Golem and Dragon, for instance) can reveal when one of your teammates might want to go collect an easy kill.
3: Targetting. Leaving a ward in the bushes in your lane will allow you to use non-skillshot abilities on the champions. BE WARNED: This will alert players to the fact you have a ward there if they have half a brain. But it may be worth it to land a CC or high-damage burst on them to kill them.
4: Stealther-discouragement. If the opposing team has a stealth ganker, a vision ward placed early may discourage them from trying to gank in your lane. Shooting a few times at them while they are in stealth, or making sure the opposing team sees you placing the ward will alert the enemy to the fact that you can see him. This will - in many cases, but not all - lead the stealther to switch to another lane, as he perceives his chances of getting a kill in your lane as being slimmer.
5: Movement. As you may have noted, I underlined friendly unit in the ward basics section. This is because a ward counts as a minion in every respect. This means you can place a ward in a bush, and allow yourself or your team to use the summoner spell Teleport to go there. Once you've tried entering a bush in one lane with blitzcrank, then teleporting to a ward in another lane for a surprise rocket fist gank, you'll want to try this over and over.
6: Ability-activation. Some champions have abilities that can target a friendly unit, and may benefit greatly from having strategically placed sight wards. Gangplank can shoot a ward to Raise Morale when your team is attacking the Baron, for instance. And Katarina can shunpo to them. (In fact, I frequently ward non-standard parts of the jungle with Katarina for Shunpo-juking).
3: Ward placement
As mentioned earlier, you will want to place your wards in bushes whenever possible. This provides you with vision inside the bush as well as outside of it, as well as minimal protection from roaming champions with oracles elixir. I've attached a map of common warding locations.
1: Bushes mid lane: Usually placed by solomid champion to warn them of incoming ganks.
2: Rivers. Provides your team with early warning of enemy champs switching lanes. Duals as a spotter for wether or not the enemy is doing dragon/baron.
3: Top/bottom lane river-bush. Like 1: This is the #1 preferred position to gank top/bot lane from if all towers are up.
4: Top/bot lane side bush. Typically used if the enemy team has a stealther. May also be used if the opposition likes to hide squishy champs.
5: Buff-creatures. Handy to gank Shaco, Warwick, Fiddle and other champions who are fond of going into the jungle for buffs
6: Midlane tower bush. Popular places for teams to stack up to gank people when pushing second tower
7: Popular top/bottom bushes to gank/towerdive from when pushing
4: The true costs of warding
Sight and vision wards have a cost of 90 and 150 gold respectively. This means that warding the entire map is an expensive option; Especially given the relatively short duration of a ward. Additionally, placing a ward in river/jungle incurs travel time where you are not farming gold and xp, and where you may be vulnerable to enemy team ganks. As such, warding is ideally a burden shared across your entire team, instead of resting on a single champion. Your carry in paticular will be hard pressed to spare the gold for early warding, although he may have a healthy surplus of gold to drop a few late-game. Likewise, champions who are pushing in one part of the map should not reasonably be expected to cover the opposite side. It will be very difficult - especially in the solo queue - to convince other players to fork over gold and time to help you ward. As a rule of thumb, if placing a ward earns your team a kill or prevents the enemy team from getting one, the ward will have earned itself back.
5: Ward-like units
Several champions have abilities that create Ward-like units on the map. These have their own advantages and disadvantages. If you see any of the following champions on your or the enemy team, these are worth considering.
1. Heimerdinger: Can place turrets. These will persist until he dies. They provide limited LoS. You can teleport to them and use abilities on them. (Heimer will probably be upset if you're Gangplank and raise morale off of his red turrets). Heimer can maintain a limited number of these, and will likely not place them outside the 3 lanes. The turrets autoattack enemies, and will therefore alert the opposing team to their presence.
2. Teemo: Can place mushrooms. These will persist for a long time. They provide VERY limited LoS. You can teleport to them and use abilities on them. Teemo is likely to scatter these all over the map. Their main way of warning your team is by disappearing when an enemy stumbles into one and they explode. If placed out of the way, the enemy is very unlikely to trigger and discover these.
3. Shaco: Can place Jack in the Box'es. These last up to 45 seconds. They provide no LoS unless triggered. You can teleport to them and use abilities on them. Shaco is unlikely to place these unless to use them within the next few seconds. If an enemy wanders near them, they reveal themselves and disappear after 5 seconds.
4. Nidalee: Can place traps. These provide NO LoS unless triggered. You can NOT use teleport or abilities on them. Nidalee is likely to scatter these in chokepoints to alert you to the presence of ganking enemies.
5. Twisted Fate: Can reveal the entire enemy team with his Ultimate. He's likely to use this often, but not necessairely when you need it.
6. Anyone with Clairvoyance: This is not a commonly picked Summoner spell. But it can double as a dragon/baron/buff spotter or an anti-gank spotter. It requires some intuition to know when and where to use this skill, but it has a fiarly low cooldown. The spell has a distinct sound effect and places a visible particle, alerting the opposition to the fact that you've used it if they're nearby.
6: In closing
So this is warding. You are more likely to see them employed to their full capabilities in High-ELO games than low-ELO. Used well, they can give you a great advantage in the early-mid game. They may (they probably WILL) win you matches if used well. If used thoughtlessly, they're likely to waste more time and money than they gain you. I've probably missed a bunch of succint points in this early iteration of the guide. Hopefully, orhter people will chime in below with their own experiences and advice on how to best utilize this vastly underused tool in your arsenal.
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