How do AirPort Express and wireless access point relate? I was reading a story about how some guy reverse engineered AirPort Express and can't get one thing: what's that AirPort Express in plain words?

I understand that there's a device called "wireless access point" that connects to a wired network of the Internet provider (that might be Ethernet) and makes that wired network accessible to Wi-Fi or some other wireless protocol users.

Apple has a picture of a white box with a recognizable 110V plug and looks like it is AirPortExpress. What does that white box do exactly and how does it relate to a wireless access point?
The article is about reverse engineering the AirPlay part of the Airport Express device. The AirPlay feature allows iTunes to stream songs (audio) to the unit and you can connect a set of speakers or an amplifier to it via the 3.5mm stereo jack.

However the protocol (and the key) AirPlay uses is not documented, and if I want to implement the same feature on other hardware (for example make my linux gateway computer have the same functionality as an AirPlay device), I have to reverse engineer the protocol (the language both devices speak over the network) and re-implement it. 

The interesting part is that AirPlay uses asymmetrical encryption to encrypt the audio stream, and the private key is located only on the device. By obtaining this key, the last part of the puzzle is found.

After I wrote this, I have found an article, which describes it (probably better) than me.