When people talk about "soft reset" that is basically about the same as it has meant for smart phones and PDAs for years: the device reboots from scratch but nothing in non-volatile storage memory is erased.
That is what happens to the iPad no matter how you get it to reset. But the iPad doesn't have a physical reset button, or a removable battery, that often was used on other devices to do a soft reset.
And indeed there are times when the OS has crashed to the point where it either does not recognize holding down the sleep button, or is unable to display the slide to turn off prompt. So built into the boot loader is a failsafe and this bypasses the OS to do the same thing: reboots the device EXACTLY like it would for a normal shut down and turning back on. There is really no point to doing a so-called "hard" reboot unless you can't get "slide to power off" to come up.
Doesn't mean that all Apple reps know this much about how it works. Since "hard" reboot works in places where a "soft" reboot won't they will often suggest that as a magic cure all even if the device is perfectly capable of doing a normal shut down.
So this has been referred to as a "hard" reboot--even though the result is exactly the same as a "soft" reboot.
Hard reboot on some other devices is quite a bit different: that is akin to bringing the device back to factory fresh.... you lose everything. That is more akin to a "restore" on the iPad.
Well now this is all clear. As mud. lol
Guys, a word of caution which I have posted elsewhere before.
A reset (regardless of whether you want to call it hard or soft) is NOT SAFE so please don't consider it or recommend it to people as in the same category as the other things mentioned in this post such as sleep, respring, and reboot, which are safe to do.
A reset is done at the bios level and therefore risks corrupting files because IOS is given no opportunity to close files it is writing in a safe manner. It is exactly the same as holding down the power button on your laptop or desktop for 10 seconds (if you don't know what this does, I do not recommend you try it!)
I've met people who held both buttons down at least once a day because they thought they were keeping their iDevice nice and healthy by rebooting it!
A reset is only to be recommended when you need to get into DFU mode or when the iPad is frozen and nothing else will work. Otherwise, steer well clear.
Hope that helps.
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However, I would never recommend doing it just as a way to reboot the device. If you can get slide to work there is NO NEED to hold down the sleep/wake button.... it is giving you the same reboot as slide to turn off.
DFU (Device Firmware Update) mode is a whole different animal though. That is yet another safety net which is why, barring a true hardware failure, there is no such thing as a bricked iOS device: you can always use DFU mode to restore it. DFU mode is in the non-writable part of the firmware so it will always be around if needed.
Speaking of bricked.... yet another term being misused due to iOS... lol.
Oh yea, might as well bring up Recovery Mode too... That is actually at the OS level and is why, if you got into it erroneously--ie, you don't really have a problem with the firmware--you can usually kick the device out of it.
Over the years of iDevices I have never had any issues with a soft reset. I even have a friend that used this method to shut down his phone. As stated below it's not recommended but it's nothing to worry about when you must do it.
(I'm going to use the term soft reset because as also stated below a hard reset is a restore)