Thats the main thing, its an OS. On the computer market you have Windows, Mac, Linux, etc/
Phone world you have Win mobile, Apple's OSX mobile (iphone), blackberry has its own os, think of it like HPUX or such where its only on specific HP hardware. Android is a Linux Kernel running a Java Virtual machine.
The main difference between Android and the others is Android is open source, you can go download the source, modify it and submit it back to the project.