Soft Reset
The soft reset is a simple restart or reboot, which is useful if some of the apps aren’t responding properly.
How to reboot and reset your iPhone X or iPhone 8
iPhone X and iPhone 8 reassign the double press and hold to Emergency SOS, since it's a simpler, easier button combination that'll better hold up to the stress of emergency situations. But that means moving reboot to a new and more complicated button combo.
- Quickly click and release the Volume Up button.
- Quickly click and release the Volume Down button.
- Press and hold the Side button (sleep/wake, on/off).
- Keep holding down the Side button, even after the Shut Down screen shows up.
How to reboot and reset your iPhone 7
iPhone 7 doesn't have a physical Home button but a capacitive, Force Touch Home button. So, rebooting functionality has been moved to the volume down button.
- Press and hold down the On/Off button on the right side
- While continuing to hold the On/Off button, press and hold the volume down button on the left side of your iPhone.
- Hold both buttons as the screen turns off, and keep holding them until the screen turns back on and displays the Apple logo.
How to reboot and reset your iPhone 6s or earlier, or iPad
- Press and hold down the On/Off button on the right side (iPhone 6s or iPhone 6) or top (all other iPhones, iPods, and iPads) of your device.
- While continuing to hold the On/Off button, press and hold the Home button on the front of your iPhone, iPod touch, or iPad.
- Hold both buttons as the screen turns off, and keep holding them until the screen turns back on and displays the Apple logo.
Resource: How to reboot and reset your iPhone or iPad | iMore
Hard Reset
The hard reset erases all the information on your phone, restoring it back to its factory state.