Soft reset deletes nothing, simply hold pwr & vol down until the buzzer goes off and release, phone should then do a proper "reset", think of it as hitting reset on a computer. Normal restart is not a reset, it will resume some things with a restart, with a soft reset you force a "proper restart from scratch" or in other words a cold boot
A hard reset on windows phones relates to doing a factory reset, i have no idea why this is called a hard reset instead of factory reset (as it is on any other device...) and this will often delete data, how much i do not know since i have yet to do one but i suspect most of it.