This post is a simple overview of namespaces you will run into when working with OData in the ATOM format. The list is not comprehensive, but you can get pretty far with it.
This is the Atom namespace, defined in RFC 4287. It's used for all "regular" ATOM things like entry, feed, link, content, etc.
This is the Atom Publishing Protocol namespace, defined in RFC 5023. It's used for the service document elements: workspace, collection, etc.
This is the 'data' namespace for OData, defined in MC-APDSU. It's used for property elements. Typically associated with a 'd' prefix, sometimes it'll be the default namespace.
This is the 'metadatadata' namespace for OData, defined in MC-APDSU. It's used for etag, type and null attributes and the count, properties and inline elements. It's also used for all metadata extensions in CSDL, such as MimeType or the FC_* attributes. Typically associated with an 'm' prefix.
Not actually an XML namespace, this is used as a prefix in 'rel' attributes in links to indicate that SOMETHING is a linked entity.
Not actually an XML namespace, this is used as the category scheme that identifies that a category term should be interpreted as the entity type name for an entry.