Being able to find your enemy is one of the keys to securing victory in World of Tanks. It's important to understand that not everything taking place on the battlefield will be visible to you at all times. Enemy vehicles will only appear on the battlefield and mini-map while you or an ally has "eyes" on them. If that vision is lost, such as when moving behind a hill, building, or line of trees, the enemy vehicle and their icon will vanish from the battlefield and the mini-map until spotted again. Understanding the interactions of the various spotting mechanics will allow you to develop a greater sense of map awareness.
The further away a vehicle is the harder it is for you to spot. Even a vehicle out in the open will not be spotted by you (revealing it on the battlefield and mini-map) if it is too far away. Until the radius of your maximum spotting distance (not be confused with viewing distance which is the maximum range at which tanks and other objects are rendered), determined by your turret, crew skills, and equipment, overlaps the enemy vehicle, the enemy will remain completely invisible.
Vehicles hiding behind cover (bushes, walls, buildings, rocks, hills, etc.) are more difficult, but not impossible, to spot. A tank idling behind a bush that completely obscures its silhouette is much more difficult to spot than one sitting out in the open. A vehicle behind cover and at a great distance is even more of a challenge to spot. Moving or firing your weapon decreases the effectiveness of cover.
Allies that are currently spotting an enemy vehicle automatically share that vision with you on both the battlefield and the mini-map. The same applies to any targets you are spotting for your allies. The shared vision is transmitted via radios equipped in each vehicle. Keep in mind that radios have a limited range, so if you and your allies are too spread out, then you will not share your spotting information.
There is a maximum range from which you can view spotted targets (this range is not affected by your turret, crew skills, or equipment), it is simply the maximum range at which tank models and other objects are rendered (commonly known as draw distance). The simplest way to determine if you’re too far away is if you note that enemy vehicles appearing on the mini-map are not visible on the battlefield. You’ll need to approach these targets until they become visible.