Reading S2P Files - What Does Each Column Represent
S2P files are also called "Touchstone" files, and different versions of the specification can be found here and here.

Just before the data section of an S2P file, there should be a line that looks like this:

# Hz  S RI R 50

This tells you 


The frequency column is given in Hz (KHz, MHz or GHz are also possible)
The measurements are S-parameters (rather than Y or Z parameters)
The values are given in real/imaginary format (real in the first column, imaginary in the second). 
The characteristic impedance is 50 ohms


Instead of "RI" for real/imaginary, you might see "MA" for magnitude/angle, or "DB" for decibel/angle. Angles are measured in degrees (Ref).

If no header line is given, the default format is frequency in GHz, S-parameters, and magnitude/angle format.