The core storage layer in Ceph is the Reliable Autonomous Distributed Object Store (RADOS), which, as the name suggests, provides an object store on which the higher-level storage protocols are built. The RADOS layer in Ceph consists of a number of object storage daemons (OSDs). Each OSD is completely independent and forms peer-to-peer relationships to form a cluster. Each OSD is typically mapped to a single disk, in contrast to the traditional approach of presenting a number of disks combined into a single device via a RAID controller to the OS.
The other key component in a Ceph cluster is the monitors. These are responsible for forming a cluster quorum via the use of Paxos. The monitors are not directly involved in the data path and do not have the same performance requirements of OSDs. They are mainly used to provide a known cluster state, including membership, via the use of various cluster maps. These cluster maps are used by both Ceph cluster components and clients to describe the cluster topology and enable data to be safely stored in the right location. There is one final core component—the manager—which is responsible for configuration and statistics. Because of the scale that Ceph is intended to be operated at, one can appreciate that tracking the state of every single object in the cluster would become very computationally expensive. Ceph solves this problem by hashing the underlying object names to place objects into a number of placement groups. An algorithm called CRUSH is then used to place the placement groups onto the OSDs. This reduces the task of tracking millions of objects to a matter of tracking a much more manageable number of placement groups, normally measured in thousands.
Librados is a Ceph library that can be used to build applications that interact directly with the RADOS cluster to store and retrieve objects.
For more information on how the internals of Ceph work, it is strongly recommended that you read the official Ceph documentation, as well as the thesis written by Sage Weil, the creator and primary architect of Ceph.