The rifles used are .22LR, a 130-year-old cartridge design that is the most sold type in the world, designated as the only acceptable round for Olympic shooting competition of any kind.
The rifles at the top level are designed for biathlon, with variants for the different categories. According to its manufacturer, the most popular rifle by far is the Anschutz 1827F, and further description of the details are provided by AccurateShooter.com.
They include adaptations to make carrying the rifle easier during the ski portion (lightweight and compact parts, aerodynamic magazine storage) and reduce impact on aiming while firing (straight pull action to reload, multiple magazine releases).
Yes, they are legitimate firearms and yes, they can injure or kill. Regulations are very strict on safety.