The attributes as handled by
chattr on Linux and some of which can be stored by quite a few file systems (ext2/3/4, reiserfs, JFS, OCFS2, btrfs, XFS, nilfs2, hfsplus...) and even queried over CIFS/SMB (when with POSIX extensions) are flags. Just bits than can be turned on or off to disable or enable an attribute (like immutable or archive...). How they are stored is file system specific, but generally as a 16/32/64 bit record in the inode.
The full list of flags is found on Linux native filesystems (ext2/3/4, btrfs...) though not all of the flags apply to all of FS, and for other non-native FS, Linux tries to map them to equivalent features in the corresponding file system. For instance the simmutable flag as stored by OSX on HFS+ file systems is mapped to the corresponding immutable flag in Linux
chattr. What flag is supported by what file system is hardly documented at all. Often, reading the kernel source code is the only option.
Extended attributes on the other hand, as set with
attr on Linux store more than flags. They are attached to a file as well, and are key/value pairs that can be (both key and value) arbitrary arrays of bytes (though with limitation of size on some file systems).
The key can be for instance:
system namespace is reserved for the system (you wouldn't change the POSIX ACLs with
setfattr, but more with
setfacl, POSIX ACLs just happen to be stored as extended attributes at least on some file systems), while the
user namespace can be used by applications (here
rsync uses it for its
--fake-super option, to store information about ownership or permissions when you're not superuser).
Again, how they are stored is filesystem specific. See WikiPedia for more information.