Buffer headers are stored in the Shared Pool (although the buffers reside in the buffer cache).
(Note: as per Oracle Insights by Cary Millsap et al, the buffer headers are in the fixed SGA (p. 197)).
The buffer header is the primary data structure that is used for buffer cache management: it relates (memory) buffers with their actual (disk) data blocks. A buffer header also stores the last time that a data block was read from disk.
Some information about each buffer header is exposed in v$bh (and x$bh).