Fairing

• Can they be used as connection for struts?

Fairings are parts which encapsulate a payload to shield it against aerodynamic stresses and heating. It also improves the aerodynamic shape reducing drag and making it more stable, if the payload is asymmetrical. The stock fairings are not real parts after separation so that they don't contribute to the number of parts in orbit as debris.

Do not worry if your spacecraft leans through the fairing, it won't break anything.

They are the first and currently^[outdated] only procedural part available in the stock game.

Interstage fairings are possible. Simply move the end of the fairing to any circular part, and adjust the position until you see the "Close Fairing" text appear. Interstage fairings (or the payloads within them) can be highly unstable. The use of struts is recommended, as the end of the fairing does not provide a structural connection.