What are the differences between numpy arrays and matrices Which one should I use

Question

What are each's advantages and drawbacks?

I've noticed that either one may, if necessary, take the place of the other, so should I use both or should I stay with just one of them?

Will the program's format affect my decision? There are a lot of matrices because I'm using Numpy to perform some machine learning, but there are also a lot of vectors (arrays).