my pal tells me that its kinda useful for stuff...what is it exactly...can it be changed?

The mac address is a unique number on your computer that is used to identify your computer in a network system. With that number it restricts outside sources/connections from connecting to your computer and in case of wifi connect it prevents people from war driving your wifi signal and using it for themselves. It is very useful when setting up security protocol and of course preventing people from hacking into your computer and what not.
But heres a better definition of what a Mac address is.

The mac address is a specific hexidecimal address burned into your network card. If you have multiple network cards (like one wireless one cable) then you'll have multiple MAC addresses. MAC addresses are not changeable and given by the manufacturer. The begining of the MAC address can tell you who made it, use google to find a site that will tell you. To find a mac address you can go to command prompt and type "ipconfig /all" without the quotes. The physical address is your MAC address.

MAC address is useful from many aspects... For example, many programmers choose to use MAC address of a MAC address from an integrated LAN controller, to calculate registration algorythms, and alike... Unfortunately, there is a growing number of tools that allow you to mask and even change MAC address of a LAN card... It appears that card manufacturers choose to store MAC address and firmware together on some sort of an EEPROM, and thus, many tools just sort of flash that chip, and you can change MAC address to anything you need it to be...

But in old days, MAC's where predefined, and unchangeable, but as Saint_Michael said, experienced computer gurus knew a wy to mask (or spoof) their real MAC address, and authenticate as anyone...

Many wireless and ADSL routers have an option of blocking or allowing certain MAC's to access your network... MAC's are still a pretty good way of protection, since a user has to be on a certain level of knowledge to change it or spoof it, and those people have better things to do, than to randomly hack into peoples computers

interesting =) i only knew mac address as an individual computer's physical address which is used in networks and such for identifying it. other than using command prompt and the ipconfig/all command, is there another way to find out a computer's mac address? ^^

Well mac address or mac ip as it is also usually known is address on your computer and each piece of hardware you've got has one. Also they are usually used in connections with local area network and mostly because to prevent someone else from entering the local area network without knowledge of the administrator they got their real meaning with the wireless networks as there entrance to the system without any knowledge of administrator was common thing to happen now many wireless networks are restricted to the mac address.

OK to stop explaining it in details mac address usually is not passed on from the first router but if server is explicitly asking for mac address then it might happen that they are passed even out of some local area network and from your internet service provider. Many Java enabled chat rooms.

Also mac address can be changed it is dangerous process but if you are linux based user then you shouldn't have any problems by changing mac address. Also I would suggest that you don't try to do it yourself as you might affect firmware of the device you are trying to alter mac address.

Hello,
The NIC (network interface card) has a single unique MAC address.

You have 256*256*256*256*256*256 possible unique MAC addresses, since 4 bits can contain the range 0-15 (16 possibilities) and there are 4+4=8 bits (16*16=256 possibilities) per "octet" (XX).

This is wildly different from the number of legal MAC addresses that can be assigned to NICs, incidentally, since many more than half of these theoretical addresses are reserved.