If I ran this command
#fsck
without arguments or options. Will it delete everything?
Normally, fsck will not cause any harm, unless you try to run it over mounted filesystem.
fsck over mounted filesystem will ask you if you want to proceed. And if you force it to proceed, then it is most likely to be very destructive. I have seen other people to use fsck this way and completely thrash Linux to the point that reinstall was necessary.