What type of coordinate system?

[shanepitts]

This is a very basic question, but what type of coordinate system is this?

When is it useful to use?

e_r, e_θ, e_Φ sinθ

 

[blue_leaf77]

Those notations are typically used for the so-called (curvilinear) spherical coordinate system. It's useful when the system to be described has a pronounced spherical symmetry, for example our Earth's magnetic field, electric field due to a charges sphere, and hydrogen atom. By the way what is that $\sin\theta$ for?