The OpenStack project as a whole is designed to deliver a massively scalable cloud operating system. To achieve this, each of the constituent services are designed to work together to provide a complete Infrastructure as a Service (IaaS). This integration is facilitated through public application programming interfaces (APIs) that each service offers (and in turn can consume). While these APIs allow each of the services to use another service, it also allows an implementer to switch out any service as long as they maintain the API. These are (mostly) the same APIs that are available to end users of the cloud.
Conceptually, you can picture the relationships between the services as so:
Dashboard ("Horizon") provides a web front end to the other OpenStack services
Compute ("Nova") stores and retrieves virtual disks ("images") and associated metadata in Image ("Glance")
Network ("Quantum") provides virtual networking for Compute.
Block Storage ("Cinder") provides storage volumes for Compute.
Image ("Glance") can store the actual virtual disk files in the Object Store("Swift")
All the services authenticate with Identity ("Keystone")
This is a stylized and simplified view of the architecture, assuming that the implementer is using all of the services together in the most common configuration. It also only shows the "operator" side of the cloud -- it does not picture how consumers of the cloud may actually use it. For example, many users will access object storage heavily (and directly).
