The -sV Nmap option enables service detection mode, allowing its users to determine the version of a running service. If version detection is enabled, the results table will contain the additional VERSION column:

PORT STATE SERVICE VERSION 
22/tcp open ssh OpenSSH 5.3p1 Debian 3ubuntu7 (Ubuntu Linux; protocol 2.0) 
25/tcp filtered smtp 
80/tcp open http Apache httpd 2.2.14 ((Ubuntu)) 
9929/tcp open nping-echo Nping echo 
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel 

Service detection performed. Please report any incorrect results at http://nmap.org/submit/ . 
Nmap done: 1 IP address (1 host up) scanned in 16.63 seconds 

The amount of returned information varies, but it is very useful as a penetration tester looking for security vulnerabilities or even as a system administrator keeping an eye on your network for unusual changes. Remember that there will be services that allow you to list supported modules and obtain very detailed protocol...