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.

The lsb_release command with the -a option prints version information for the Linux release you're running. For example, typing:

lsb_release -a

will give you:

No LSB modules are available. Distributor ID: Ubuntu Description: Ubuntu 8.10 Release: 8.10 Codename: intrepid