Evidently you have two hard drives in the machine, and you have a dedicated
/home partition on the second one.
What's happened is
/dev/sdb1 (which is not your USB drive) is now mounted twice, once on
/home and once on
/mnt. The only real risk in that is you now also have double the opportunities to, e.g., delete something accidentally. If you are not in the habit of doing such things, then no worries.
How can I get out of this situation without compromising any data?
umount /mnt. If you get a "device busy" message, it's because a shell or file browser is open to that location -- change directory in those.
Where can I find an accessible explanation of how to mount a usb drive?
Contemporary linux distributions (and I presume other *nix's) come out of the box with desktop environments that will do automounting, so most newbies don't have to worry much about this stuff.
If for some reason you aren't using an automounter,
man mount is a good starting place. Device nodes get assigned in order -- this is why
/dev/sdb1 turned out not to be the USB. One way to get a clue about the proper device node would be to look at the various
sd files in
/dev without the stick in, then add it and look again. If you already have two hard drives, plugging in another (including USB sticks) will probably yield sdc, and sdc1 would be the first partition.