Hello!
When I browse throught packages in linux dvd, I found a lot of packages that contains firmware, for example, atmel-firmware.
As I know, firmware is a kind of software that works under some device, like WI-FI adapter. Why linux contains such kind of packages?

Some distributions contain them because without the firmware the device would not work. This is especially problematic to people who only have wireless internet access so can't get the firmware either way.

Why? So people can use their hardware...

Because they want it to be reasonably useful. A system that can't be set up in isolation is minimally useful to a lot of people. These "pure" distros that don't include stuff that people commonly need are not useful to someone that, for example, mainly uses laptops with wireless connections (like me). It would be impossible to get a working system in many cases without having another computer to get the things you needed before the install. Thats why some of us prefer to stick to distros that include everything they can.

As far as I can tell, the responses above me are all talking about drivers, not firmware. Firmware runs ON the device in question, drivers tell the OS how to USE the device in question. The OS only needs drivers, not firmware. Firmware is only used when you're actually reprogramming or updating the device itself, such as reflashing a new firmware for your BIOS, or updating the firmware on your SSD.

As far as why the OS DVD includes firmware, I don't know. Maybe it's actually a driver that's been mislabeled?

I first came across firmware these firmware files when trying to install a Freecom DVB Digital TV USB stick in Linux. The firmware file is loaded onto the device during boot - not flashed like BIOS. I believe there's a firmware directory somewhere in the Linux filesystem where similar files are kept.

It's like Apple's toolbox which used to contain all the code to draw windows, sliders, scroll arrows, etc. In older kit with Motorola 68000 CPUs, the toolbox was held in ROM, the OS just called up a window routine and supplied various parameters, size, etc. Windows 3.1 had to generate their windows from the OS on disk so wasn't so fast. Laterly, on PowerPC systems, Apple ended up with a Firmware file which, like the devices mentioned in Linux, was loaded into RAM on boot and accessed from there.

I can just tell you're facinated now,

Play Bonny!

So the people on the Debian project don't know what they're talking about then?
For example: http://packages.debian.org/sid/atmel-firmware

If you read the description, you'll see that that's a special case device where the firmware is located in volatile storage, which means the OS does need to reload the device with its own firmware every time it's powered up. This is what Soadyheid was referring to in his post, and is different than the OS drivers for the device.

This is by far the exception, not the rule. I didn't even realize that devices like this still existed, but it does explain/answer the OP's question.

My apologies, my post was a little rude, I was just a bit miffed at being misunderstood to have misunderstood.
My first post was written referring to just this type of situation as I have seen it many times for wireless cards and a at least a couple of times for other devices (one was a TV card the other[s] I don't recall).

I didn't realize those were the cases you were referring to in your original post, my apologies.