To add to what Roger posted above, most of the time marketing departments refer to one billion bytes as a 'GB', but in reality, as reported by the system, a gigabyte is actually 2^30 bytes, which is 1,073,741,824 bytes.
So if a storage device claims 32GB of storage, it likely has 32 billion bytes of storage, maybe slightly more. When the system sees that much storage, it will report it using the "binary" definition of GB (1 GB = 2^30 bytes) and 32 billion bytes will report as 29.80 GB.
So really the 32 GB storage as reported by manufacturers is 29.80 GB to "the system". Add to that the system software and apps that come with it and that's why you'll see less than that reported as available; in the case of the OP, 26.5GB.