The majority of vdevs will be used for storage, but there are also special kinds of vdevs:
LOG vdevs (the SLOG) is a fast persistent write cache for ZFS. It may provide performance improvements, but generally, it is implemented primarily to protect data in the event of power failure.
CACHE vdev (usually called the L2ARC) is a read-cache. Reads are cached in RAM (L1ARC or simply ARC), but adding an L2ARC allocates storage space (generally on fast media like SSDs or Optane NVMe drives) for additional caching capacity to improve read performance on frequently accessed files. This can provide a negligible to large impact on performance depending on the types of files stored on the ZFS server.
For more detailed information on how ZFS caches data, check out this article on ZFS caching
. In general, ZFS offers a lot of robustness in how you wish to tailor your systems caching to your use case – another area where being "software-defined" aids ZFS's robustness.
SPECIAL vdevs store metadata for ZFS systems. Special allocation class vdevs can provide a strong performance increase by using SSDs as your devices to quickly fetch file metadata within your zpool (the performance benefit will depend on the use case). The required size of your metadata vdevs will depend on your size of storage vdevs. These can be especially useful for applications with a heavy metadata workload, such as the VEEAM backup repository.