Scanning is a process of probing target with intend to reveal useful information that can be used for other stages of attacks.
Scans can be broken into 3 different categories.
Port Scan
Network Scan
Vulnerability Scan

Port Scan
Port scanning process used carefully crafted packets and messaged which being send to target in order to get more information regarding ports and what is listening behind them. Most of this scans associated with known port numbers below 1024. One of the most popular tools to use for port scanning is nmap.

Network Scan
With network scanning we look to discover all hosts available on the network. One good example of network scan would be ping sweep, which scans ip range and determines weather host is alive. Example of tools that can be used are nmap and Angry IP.

Vulnerability Scan
Vulnerability scan identifies weakness and vulnerabilities on a target system. Typical vulnerability scan will discover hosts, access points, open ports, threats. Below you will find some examples of popular vulnerability scanners.
Tenable’s Nessus
Rapid7’s Nexpose
Burp Suite
Nikto
WebInspect

The following information can be reviled during scan.

Live hosts present on the network
Information on open/closed ports
Operating system
Services and processes running
Vulnerabilities
Firewalls
Patches
IP addresses