There are four levels: raw disk, raw partition of that disk, formatted filesystem on a partition, and actual files stored within a filesystem.
/dev/sda means an entire disk, not a filesystem. Something with a number at the end is a partition of a disk:
dev/sda1 is the first partition of the
/dev/sda disk, and it's not even necessarily formatted yet! The filesystems each go on their own partitions by formatting each partition with its filesystem.
So, what will generally happen is that you'll partition
/dev/sda1 with a filesystem, mount
/dev/sda1's filesystem to somewhere, and then begin working with files on that filesystem.
Why have a unified filesystem
Linux (and UNIX in general) has the concept of the virtual filesystem. It combines all your real disks into one unified file system.
This can be quite useful. You might, for example, want to put your operating system and its programs on one really fast real disk and all the users' personal files on another fairly slow but huge disk because you want the OS to be fast but you want an affordable means of handling the files of thousands of users.
Unlike the usual method in Windows, which by default breaks each disk up into a separate letter and where using
D:\Users might break some programs that hard code the path
C:\Users, this can be done with ease and fluency. You format one partition in each disk, you mount the OS one to
/ and the user one to
/home, and it acts like a system that put everything on one real disk, except you get that speed and affordability tradeoff you wanted.