Depending on what you want to see, there are several options on Terminal level:
• top: show running processes, memory usage and similar stats
• iostat: show I/O per terminal, device and SPU summery statistics
• vmstat: show Mach virtual memory statistics
• df and diskutil list: report on drive space used and free
• fsusage: show file activity for both disk and network
• nettop: display updated information about the network (a bit like top for net I/O)
• w: display who is logged in, what they are doing and system load
• ifconfig and ipconfig: network interface and IP protocol details
Most of these commands have a huge list of options, it’s probably best to consult the man pages for details.
I usually rely on top -u -s 10 to identify CPU-hogging processes and fsusage -f filesys/fsusage -f network to identify processes generating a lot of disk/network load.