What is CalculiX?
CalculiX is two programs
CCX - solver
CGX - Pre-PostProcessor
CCX looks like as one binary file (in Windows it is ccx.exe). If you will try to run it as usual (by double-click) nothing will happen. CCX is console application with text file interface. At first you should create text file (File.INP) with mesh, named groups and input (boundary conditions) applied.
After that you can run solver from terminal emulator "ccx file". CCX will read text file and make calculation. The result will be many text files stored at the same folder. For example .FRD file will contain all results (as you will order in File.INP)
What is CGX? CGX is pre-postprcessor. What you can do with CGX?
You can run input file for preprocessing (to see and check groups of nodes, element faces and elements).
You can apply some boundary conditions (for example pressure or film coefficient for convection) with using "send" card in CGX.
When calculation is finished, you can run .FRD file for the postprocessing (to check stress, displacements and etc).