How radio communications in WoT work (or: How to "see" opponents outside your own visual range):
In World of Tanks, your vehicle has a limited view range at which it can spot other vehicles (friendly or enemy). This view range is affected by your turret, the other vehicle's profile, camouflage, whether it's moving or firing, etc. However, an important mechanism is the use of radio to send and receive information on the location of other vehicles, allowing for situational awareness beyond your own visual range.
As an example, you can be looking down a long street from your tank destroyer and not see any enemies. However, when a friendly tank moves down the street, it spots an enemy tank that was there all along. You will then see the tank just as if you spotted it yourself, as long as the friendly tank can see it, and is within radio range of you.
The game mechanics of radio communications:
All vehicles have a radio module. The primary characteristic of this module is its range. Two friendly vehicles can communicate if they are closer to each other than the sum of their respective radio ranges. Thus a tank with a 300m range radio and a tank with 500m range can communicate up to 800m away.
All communication is thus a 2-way relation between two vehicles: If you can communicate with a friendly vehicle, then you will know about any vehicles that he can see directly (including himself), and he will know about any vehicles you see directly. However, there is no chaining, or relaying of information - he can only tell you about any vehicles that he has spotted himself, not any that he knows about from radio communications with others.
In total, then, you will know the location of
1) Any vehicles within your visual range, friend or foe.
2) Any friendly vehicles within your radio range to them (for each vehicle this range is the sum of your respective radio ranges).
3) Any vehicles (friend or foe) within the visual range of the vehicles in 2), friend or foe.
An example figure to show how it works: