Each current element $I\,d\vec l$ produces, at a point located by vector $\vec r$, a differential field of magnitude $dB = (\mu_0/4\pi)\,I\,dl\sin\theta/r^2$, directed perpendicular to the plane containing $d\vec l$ and $\vec r$ by the right-hand rule. This is the 'Coulomb form' of magnetism: contributions fall as $1/r^2$ and are weighted by $\sin\theta$ (an element parallel to $\vec r$ contributes nothing).