Free or "libre" software is software that obeys the Four Freedoms. Much "open source" software is also Free software, but not necessarily all of it.

I'm not sure if this is still the case, but for a while Microsoft would provide source-code to the Windows NT kernel to universities and academic institutions; the source was open (for some definition of "open") but because Microsoft did not allow redistribution and modification, and only allowed the code to be used for educational purposes, it was not Free software.