A MAC address is a unique identifier assigned by the manufacturer of a network interface controller (NIC) to interfaces. The first three octets of the address identify the organization that issued the identifier and are known as the Organizationally Unique Identifier (OUI).
This means that if there is a device on the same LAN you are on, and you were able sniff the traffic coming of that device, you will be able to identify the maker of that device, whether it is an Applie iPad, Dell Laptop or Cisco Router.
Wireshark Packet Sniffer - formerly known as Ethereal - is a must-have tool for every Network Engineer. And they have an OUI Lookup Tool that you can access from this link.