In its standard mode, nmap does two different types of scan: a host scan, to determine which hosts are available for further port scanning, and a port scan, which reveals the status of ports on available machines.
-sn does no port scan, but it does a host scan -- this is particularly useful when scanning a range with nmap, where it will print out those hosts that responded to the scan (which could, for example, reveal some of the hosts available on a certain subnet).
$ nmap -sn 192.168.0.0/24
Starting Nmap 6.40 ( http://nmap.org ) at 2013-08-23 17:54 CEST
Nmap scan report for 192.168.0.11
Host is up (0.051s latency).
Nmap scan report for 192.168.0.37
Host is up (0.063s latency).
Nmap scan report for 192.168.0.65
Host is up (0.016s latency).
Nmap scan report for 192.168.0.85
Host is up (0.00090s latency).
Nmap scan report for 192.168.0.149
Host is up (0.024s latency).
Nmap scan report for 192.168.0.202
Host is up (0.021s latency).
Nmap scan report for 192.168.0.253
Host is up (0.042s latency).
Nmap done: 256 IP addresses (7 hosts up) scanned in 22.26 seconds