What is MAC address?
MAC address (Media Access Control address) - is a unique identifier of a network interface (usually hardware network card) for communication on the network physical level.
Actually it is a unique number assigned to a network card by a manufacturer and is stored in a hardware's read-only memory.
The MAC address length is 48 bits, which gives 248 (which is equal to 281,474,976,710,656) of possible addresses. Usually representation of MAC address is written as six groups of two hexadecimal digits separated by "-" or ":". So it is look like "00:11:22:33:44:55" or "67-78-89-AB-CD-EF".
To see a MAC addresses of the network interfaces on your PC is just required to type "ifconfig" command in Linux, "ipconfig /all" command in Windows or "networksetup -listallhardwareports" on Mac OS. It will be listed for each interface as "HWaddr", "Physical address" and "Ethernet address" accordingly.
Generally, MAC address is assigned permanently to the device and can not be changed. But in some circumstances there is a possibility to change MAC address on a software level. It is known as MAC-address spoofing. Any way, it is subject only to a local broadcast domain.