Description of motion along a straight line the role of direction is played by +ve & -ve signs of that direction, however to describe motion in 2 & 3 dimensions we need vectors.
^What is a vector
If both magnitude and direction are required to completely described a physical quantity, then it is called a vector. A vector quantity is represented by putting on arrow above it or by bold letter e.g. it Q is vector then we represent it as or Q. If a quantity can have any direction it is called polar vector. If its direction is along axis (axis of rotation), then called axial vector.