A BLOB (Binary Large OBject) is a chunk of bytes of an arbitrary length. It can be a single file, a segment of the file or a piece of metadata.
Since the goal of this project is to provide content-addressable storage, Blobs in the system are immutable. For mutable objects, see Pins.
A Ref is cryptographic hash of a Blob content. It consists of two parts: the hash algorithm and the hash payload.
For example, SHA256-based Ref may look like sha256:fee8292... in a
text encoding.
A schema blob contains a metadata about other blobs or their relations. In current version, schema blobs are stored as JSON objects with a specific formatting, so they can be distinguished from other JSON blobs.
As an example, a schema blob may describe a Unix file with a name, a size and permission bits and points to a blob with actual file content.
Pin is a mutable named variable that points to a single Ref. Since a Ref may point to a schema blob that describes a file, a directory or any other object, it is possible in general to store arbitrary mutable objects in CAS.
This concept is similar to a branch in Git or a permanode in Perkeep.