OpenStack is a collection of open source software projects that enterprises/service providers can use to setup and run their cloud compute and storage infrastructure. Rackspace and NASA are the key initial contributors to the stack. Rackspace contributed their "Cloud Files" platform (code) to power the Object Storage part of the OpenStack, while NASA contributed their "Nebula" platform (code) to power the Compute part. OpenStack consortium has managed to have more than 100 members including Canonical, Dell, Citrix etc. in less than a year.
OpenStack makes its services available through Amazon EC2/S3 compatible APIs and hence the client tools written for AWS can be used with OpenStack as well.
There are 3 main service families under OpenStack
Compute Infrastructure (Nova)
Storage Infrastructure (Swift)
Imaging Service (Glance)
