The df command displays filesystem disk space usage for all partitions.
df -h
will give information using megabytes (M) and gigabytes (G) instead of blocks (-h means "human-readable").
The free command displays the amount of free and used memory in the system.
free -m
will give the information using megabytes, which is probably most useful for current computers.
The top command displays information on your GNU/Linux system, running processes and system resources, including CPU, RAM & swap usage and total number of tasks being run. To exit top, press q.
The uname command with the -a option, prints all system information, including machine name, kernel name & version, and a few other details. Most useful for checking which kernel you're using.
