From charlesreid1

The Basics

Gnuradio is a software program that provides digital signal processing. When coupled with a receiver and/or transmitter, this enables all of the complex signal processing, modulation, manipulation, and processing to happen in software - cheaper and easier than building a new circuit every time you want a different filter.

Gnuradio vs Gnuradio Companion

Take note: gnuradio is a software library that provides signal processing functions. This is not the same as the GUI window with blocks connected with arrows. That's Gnuradio Companion - a GUI interface for easily interacting with the Gnuradio library. But this is just one way of interfacing with the library. You can write libraries to process signals entirely outside of the Gnuradio Companion, by using the Gnuradio software library - e.g., a script that is entirely in Python. Gqrx is another example of a program that uses Gnuradio under the hood, and provides an alternative GUI interface.