Reaction Control System
The Reaction Control System, mostly just called RCS, is a set of monopropellant-fuelled small thrusters primarily intended for orienting vessels in vacuum. It is toggled on and off by pressing the R key (default key binding). In order to use RCS, a dedicated RCS fuel tank along with some RCS thrusters are needed. The fuel tanks for RCS are different from solid or liquid fuels tank, and currently come in three options, two matching the two different radii of parts, and one side-mounted type.
The thrusters can be located anywhere on a ship. Unlike Jet and Liquid Fuel Engines, there is no need to place RCS thrusters on RCS fuel tanks or manually run fuel lines to them. The thruster block can provide full rotation control, whereas the linear port can only provide thrust in one direction. It is advisable to place the thrusters as far away from your centre of mass as possible, thus maximising the torque (turning force) they generate. It is typically most practical to place them with four way symmetry as this can provide the best control. In theory, you can provide suitable roll, pitch and yaw control with just one set of four way thruster blocks. Translation manoeuvres however are much easier with one set at the top and another one at the bottom of your rocket. Activating the SAS will prevent unintentional changes of heading during translation movement.
It should also be noted that the SAS system will make use of the thrusters if you have an ASAS Module fitted to your craft. To avoid a constant drain of RCS fuel, it is recommended to not have both the RCS and the SAS systems enabled at the same time, unless you need to keep your ship dead level and/or have RCS fuel to spare.