Live processes, refreshing on screen
ps gives a snapshot. top and friends give a live view with sortable columns and quick keys to kill or renice processes.
top (built-in)
Run top. Inside: q to quit, ? for help, k + PID to kill, P sort by CPU, M sort by memory. macOS top is BSD-flavored; some keys differ from Linux top — read the inline help.
htop — friendlier UI
brew install htop. Color, mouse support, tree view (F5), filter (F4), kill menu (F9). Same data, much easier to drive. The default for most engineers.
btop — modern eye-candy
brew install btop. CPU/memory/disk/network graphs side by side. Pretty and informative; some find it slower than htop on lots of processes. Worth trying once.
Specialized monitors
nvtop— GPU usage (Linux/Nvidia).iftop— network per-connection.iotop— disk I/O per process (Linux).