"These instances are Linux®-based and can run any
application or software you want. You are in control of each
instance. The EC2 environment itself is built on top of the open
source Xen hypervisor, which was initially developed at the
University of Cambridge. Amazon lets you create Amazon machine
images (AMIs) that act as the templates for your instances. Access
to the instances can be controlled by specifying the permissions.
You can do anything you want with them; the only restriction is
that they need to be Linux-based images. Recently, Open Solaris
support was announced by Amazon in a partnership with Sun, but the
vast majority of the free and commercially available pre-built
images for EC2 are based on Linux."