There are two kinds of shoulder-launched weapons. The first is the recoilless gun, which is essentially an open tube. When fired the reaction gases (with a momentum equal to the projectile) expelled out of the back of the weapon compensate the force exerted on the projectile.
The other type is the rocket-propelled grenade; these typically also use a small recoilless charge or compressed gas system to get the projectile out of the barrel and to a distance where the operator will not be hurt by the rocket's backblast; when the rocket ignites at a safe distance, it further accelerates the projectile or at least keeps it from decelerating in its trajectory.
Shoulder-launched weapons typically fire at one of two main target types — ground targets or air targets. Weapons for use against ground targets come in a wide variety of types and sizes, with smaller, unguided weapons generally used for close range combat and larger, guided systems for longer ranges. Most of these weapons are primarily designed for anti-tank warfare, although they are also effective against structures and a number of weapons have been designed specifically for such targets.
Anti-aircraft weapons, known as man-portable air-defense systems (MANPADS), are small surface-to-air missiles. They are typically infrared homing weapons and used to target helicopters, UAVs and other low-flying fixed-wing aircraft.