Linux Kernel: What is the major difference between the buffer cache and the page cache?

The page cache caches pages of files to optimize file I/O. The buffer cache caches disk blocks to optimize block I/O.

Prior to Linux kernel version 2.4, the two caches were distinct: Files were in the page cache, disk blocks were in the buffer cache. Given
that most files are represented by a filesystem on a disk, data was represented twice, once in each of the caches. Many Unix systems follow a similar pattern.

This is simple to implement, but with an obvious inelegance and inefficiency. Starting with Linux kernel version 2.4, the contents of the
two caches were unified. The VM subsystem now drives I/O and it does so out of the page cache. If cached data has both a file and a block representation—as most data does—the buffer cache will simply point into the page cache; thus only one instance of the
data is cached in memory. The page cache is what you picture when you think of a disk cache: It caches file data from a disk to make subsequent I/O faster.

The buffer cache remains, however, as the kernel still needs to perform block I/O in terms of blocks, not pages. As most blocks represent
file data, most of the buffer cache is represented by the page cache. But a small amount of block data isn't file backed—metadata and raw block I/O for example—and thus is solely represented by the buffer cache.

See also my answer to What
is the difference between Buffers and Cached columns in /proc/meminfo output?