Old DOS-style visible device names are not meaningful in Unix-like filesystems, which present a unified tree view.
A device (/dev/hda1, /dev/hda2, et cetera) can be mounted anywhere. On one of my systems, /home is /dev/hdb1, but I don't have to know that. I just refer to /home, and if next week it gets changed (because I bought a third big hard drive), nothing is really different from an end user perspective (just a lot more space available). /home is always /home.
It's customary to use /mnt to mount temporary, removable devices, such as cdroms, floppies, et cetera. But they could be mounted anywhere.
If you run the "mount" command with no args, it will report on what physical devices are mapped to what nodes in the filesystem tree.
Remote filesystems also can be mounted anywhere, through smbmount (Microsoft-style CIFS shares), nfsmount (Sun's Network File System), and similar utilities for Netware and Appleshare. In other words, you can mount file space that is in a Windows or a Macintosh elsewhere on the network. And it just looks like the Unix filesystem.