What is the temperature on the ISS (International Space Station)?

It’s apparently about 24 degrees C.
The heating system is probably both very simple and hilariously complicated depending on how you look at life. Think about it – it’s a sealed box surrounded by absolutely nothing. Inside everything (computers, lights aircon, even the astronauts) is producing heat. Outside it’s minus 247. The trick is to keep hold of the heat you produce without cooking everything.

Heat from the Sun and cold to space is controlled by insulation. Heat from the people and computers is controlled by the cooling system. Here is some info on that:
https://science.nasa.gov/science-news/science-at-nasa/2001/ast21mar_1
https://www.nasa.gov/content/cooling-system-keeps-space-station-safe-productive
https://www.space.com/21059-space-station-cooling-system-explained-infographic.html

I did a Web search and found this. This may be a bit of an oversimplification, but this is my understanding. The ISS has to avoid overheating from solar radiation when it is between Earth and sun, and it has to avoid freezing when Earth is between the ISS and the sun. To do this, the ISS is insulated by covering it with Mylar. The Mylar does such a good job that special cooling systems need to be installed inside to counteract heat generated by onboard equipment.

Also worth noting is that the ISS is like the inside of a Thermos bottle that doesn’t need an outside layer to contain the vacuum of space. So there is little or no heat loss or gain from the temperature outside. The shady side gets cold (possibly -247 degrees, if that is correct) because there is no sun and the sunny side gets hot (possibly +400 + degrees) because of the sun. So the insulation is critical, as stated by @LostinParadise. And as @RocketGuy noted, “Heat from the people and computers is controlled by the cooling system,” both top notch answers.