There are currently seven core components of OpenStack: Compute, Object Storage, Identity, Dashboard, Block Storage, Network and Image Service. Let's look at each in turn.
Object Store (codenamed "Swift") provides object storage. It allows you to store or retrieve files (but not mount directories like a fileserver). Several companies provide commercial storage services based on Swift. These include KT, Rackspace (from which Swift originated) and Internap. Swift is also used internally at many large companies to store their data.
Image (codenamed "Glance") provides a catalog and repository for virtual disk images. These disk images are mostly commonly used in OpenStack Compute. While this service is technically optional, any cloud of size will require it.
Compute (codenamed "Nova") provides virtual servers upon demand. Rackspace and HP provide commercial compute services built on Nova and it is used internally at companies like Mercado Libre and NASA (where it originated).
Dashboard (codenamed "Horizon") provides a modular web-based user interface for all the OpenStack services. With this web GUI, you can perform most operations on your cloud like launching an instance, assigning IP addresses and setting access controls.
Identity (codenamed "Keystone") provides authentication and authorization for all the OpenStack services. It also provides a service catalog of services within a particular OpenStack cloud.
Network (codenamed "Quantum") provides "network connectivity as a service" between interface devices managed by other OpenStack services (most likely Nova). The service works by allowing users to create their own networks and then attach interfaces to them. OpenStack Network has a pluggable architecture to support many popular networking vendors and technologies.
Block Storage (codenamed "Cinder") provides persistent block storage to guest VMs.
In addition to these core projects, there are also a number of "incubation" projects that are being considered for future integration into the OpenStack release.
