Few things are more important in DotA than having vision of your opponents. At its core, DotA is a strategy game and in order to make the best strategic decisions, it is key to have as much information as possible. While your team often has vision of the lanes due to the presence of creeps and towers, the lanes make up a relatively small portion of the map. Warding the other areas on the map gives your team a huge tactical advantage, but your stock of Wards is quite limited and refills very slowly. Therefore it is important to place each ward in a place that gives you the maximum sight radius. This guide is certainly not a comprehensive list of warding locations in DotA, but it covers all of the fundamental ones. Feel free to experiment and find your own useful locations!
NOTE: PICTURES OF EACH WARD LOCATION ARE COMING SOON, PLEASE CHECK BACK!
The two most important warding locations on the map are the cliffs that overlook the top and bottom rune spawns. Not only do these wards show your team what rune has spawned and where it has spawned, but they also will spot most enemies traveling from lane to lane for ganking. They boost your offense by giving your team rune control and boost your defense by spotting ganks, it doesn't get much better than that. These spots are very important to have covered early and midgame, but become far less important later on because the lategame tends to involve more 5 on 5 team battles than ganks.
Wards can be placed in the enemy forest for two reasons: preventing neutral creep spawns and spotting heroes to pick off. Early in the game it can be very beneficial to block a creep spawn, shutting off the enemy's ability to creep pull their lane. Jungling heroes are also very easy to pick off in the earlygame, so if you can spot them with a ward, it's not hard to go take them out and stunt their farming. Late in the game, the forest is generally considered to be one of the safest places to farm, so having a ward there will often give your team opportunities to pick people off and then take a tower while you have an advantage.
Near the Enemy Base
Wards near the opposing base are nearly useless for the majority of the game, but when you are trying to push for victory, they suddenly be coming very important. It's often quite hazardous to push a base, you're fighting uphill against a tower and your opponents have plenty of room to come at you from all directions. A well-placed ward will let you know how your enemies plan to defend and prevent your team from getting caught off-guard. Picking off the guy that was meant to ambush you from behind, because your team spotted him with a ward can be the play that wins the game for your team. Always try to have ward cover when you attempt a push!