Just because I find it interesting, your PC at home has one or more disks in which is called
DAS (direct attached storage) because that's what it is.
There's two things wrong with that when you get into more powerful computing.
1) You might have 50GB free on your disk when someone else is desperate for 30GB more but you can't share it
2) Your disk is limited to writing or reading one chunk of information (IO is input/output or reading/writing) so what if you want to read or write more quickly?
A SAN takes all of the disks out of the computers a business runs and merges them all together in one device called a SAN. It then subdivides everything back up into just the right amount of storage for each computer. That instantly solves number 1).
However, the great thing is if you've got 100 disks in a big device all being shared, on a simple level, it's capable of reading 100 chunks of information at once, or writing 100 or somewhere in between so read 50 and write 50.
On that level, it solves number 2 and is great for a forum where if you think about it, there are loads of reads and writes, reads are when we all view existing forum posts. Writes are when new posts are created and stored.
Hope that's slightly clearer than mud now!
And for any geeks reading, I know I've simplified but it's far too complicated in reality to discuss on a forum.