uptime prints the current time, the system's uptime, the number of logged-in users and the current load average.
If an argument is specified, it is used as the file to be read to discover how many users are logged in. If no argument is specified, a system default is used (uptime --help indicates the default setting).
The only options are --help and --version. See Common options.
For example, here's what it prints right now on one system I use:
$ uptime 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
The precise method of calculation of load average varies somewhat between systems. Some systems calculate it as the average number of runnable processes over the last 1, 5 and 15 minutes, but some systems also include processes in the uninterruptible sleep state (that is, those processes which are waiting for disk I/O). The Linux kernel includes uninterruptible processes.