A filesystem/partition like
/dev/sda1 can be seen (or at least represented as an interface in the root filesystem) as a "block device" which is one of the categories of files. Files are representations of data stored somehow, and a filesystem is a bunch of data with some kind of coherent structure. The block device file is a "raw" representation of the structured data. To make it accessible as files and directories that you can read and interact with, it can be mounted...
ls -l /dev - you see some entries starting with the letter
b indicating a block device, and this will include any filesystem such as internal partitions, peripheral devices, etc... here's some from my system (#comments added by me)
brw-rw---- 1 root disk 179, 1 Dec 3 07:14 /dev/mmcblk1p1 #ESP
brw-rw---- 1 root disk 179, 2 Dec 3 07:14 /dev/mmcblk1p2 #root partition
brw-rw---- 1 root disk 179, 3 Dec 3 07:14 /dev/mmcblk1p3 #swap
This wikipedia article explains the concept of a device file in some detail.