Numpy - numerical python, numerical arrays, mathematical operations on arrays. It is the primary data container passed between algorithms. They are more efficient and lower level languages can operate on them without copying the data.
Scipy - High level numerical routines, optmisiation, regression and interpolation
Matplotlib - 2d visualisations and interactive plots
pandas - Rich data structures and functions for working with structured data. The primary object is a DataFrame. Combines the high performance array computing features of numpy with flexible manipulation of spreadsheets and relational databases.