Filesytem on Android
There are already answers for all parts of your question, so let me sum up:


Android Folder Hierarchy has some general information
Confused by the many locations of the virtual /sdcard/ should answer the last part of your question
The /sdcard direcory and other directory conventions: how is the Android filesystem structured? might also be helpful


If you take a closer look at /sdcard from a terminal (or via adb shell) using the ls command, you will see it is a symbolic link (the Wikipedia article explains it in detail), usually created for at least two reasons:


make it easier for you to find it
have a "canonical place" for it, regardless of where it is physically


There's of course more to it, but that would become too lengthy for an answer here :)