When I was using Windows, one of my favorite apps was Glasswire so I could see every bit of data that my machine was sending and receiving and which apps were doing so. Is there a application that is similar to this? Thanks!
tcpdump if you want a lightweight, console only program.
Wireshark if you want a full fledged GUI, which can visualize and filter dumps from tcpdump, or capture all by itself.
Nethogs seems to be able to give you a per process network usage, though I never tried it. I think this should be the closest one to glasswire.