The vectors we will use we be refered to right-handed Cartesian axes.
Right handed means that the x,y,z axes are oriented in a particular way (which you must know). See diagrams.
Unit basis vectors i, j, k aligned with the x, y, z directions.
i, j, k form an ortho-normal basis (i.e. mutually perpendicular set of unit length vectors) for the 3-D Euclidean space thatwe are familiar with.