what is a file system?

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http://www.linuxquestions.org – Hi, i'm quite new to linux so i've begun studying LPI 101. i came across the following which describes hard links.. "hard links do not span file systems. this means they cannot create a link from /usr/bin/bash to /bin/bash if your / and /usr directories exist on seperate file systems" i need clarification on the following: 1. under what circumstances will / and /usr directories exist on separate file systems? 2. here is a snapshot of my root directory: admin@linux-2d4r:/> ls bin dev home lost+found mnt proc sbin sys usr boot etc lib media opt root srv (HowTos)