Being around too much of a temperature extreme for too long will cause many negative effects.
Temperature in-game is divided into three simple categories:
  1. 1.
  2. 2.
  3. 3.
You can check your current temperature by viewing the thermometer available via the /temp command.
C =====
===== 0 =====
===== H
Ideally, you should be at 0 temperature, or within the first segment on either side.
If you reach the second segment on either side, you would be either freezing (C) or overheating (H). If you remain in this temperature level for too long, you will start experiencing symptoms that will worsen the longer you remain exposed to that environment.
Each segment on the thermometer is equal to 1.
If you are overheating, you start losing health, get the sweating effect and your thirst bar will drop quickly.
If freezing though, you will become slower and receive damage.

Core temperature:

The core temperature is 0. However, depending on the armor that you are wearing ( leather or chainmail armor ), your temperature will get affected.
Your temperature can also be affected depending on whether you are exposed to the sky, or surrounded by blocks.
The higher you are, the colder it is. The lower you are, the hotter it is.
Burning spikes your temperature to the extreme.

Ambient temperature:

Your temperature also depends directly on the biome you are in, your elevation, and nearby blocks in a 2 block radius.
For example, lava, fire, lit campfires, lit furnaces, smokers, blast furnaces, all types of ice, and snow, all may affect your temperature levels.

Ailment temperature - COMING SOON:

Diseases, Injuries, and all sort of ailments have the possibility of affecting your temperature as well.

Ailment Relief - COMING SOON:

Remedies can include a certain number of seconds of hot or cold relief. When this relief is applied, the player will not suffer any symptoms from either overheating or freezing (depending on the relief type).