The document discusses the buffer cache in UNIX systems which stores frequently accessed data from disk in memory for faster retrieval. The buffer cache is organized as a pool of buffers that hold blocks of data read from disk. When a process needs to access a block, the system first checks the buffer cache and if the block is present, it is returned to the process from memory instead of reading it from disk. When a block is no longer needed, it is released back to the buffer cache where it may be overwritten or written back to disk.