"A Vector is a quantity which has both magnitude (size) and direction, but no position"

Some engines/people differentiate between vectors and points - both usually have 3 components (.x, .y, .z), but a point describes a position, while a vector a direction/magnitude.

So if you had a Point p1 = (1, 1, 1) and another p2 = (2, 3, 4) you could do p2 - p1 = (1, 2, 3) - and (1, 2, 3) is a Vector which describes "how to get from p1 to p2", so it has a direction and a size (its length/size/magnitude - calculated as sqrt(x*x+y*y+z*z) - is the distance between p1 and p2, in this case sqrt(1+4+9) = sqrt(14) = 3.7417).

idtech4 just uses the vector type for both points and vectors