Is this an insect? Which one?
It is a silverfish. It is a 'basic lineage' in the insect class. You'll find it here on OneZoom.org.

Silverfish are considered household pests although I don't think they spread disease nor cause much damage (thanks to @WilliamBligh for his comment). They typically live in the canalization (bathroom, kitchen) or anywhere that is wet enough. In a building, they may be hard to get rid of as they can easily move from one apartment to another. You have to keep your place very clean and eventually use toxins such as Permethrin or Deltamethrin.