Windows 10: What is the difference beteen uefi and legacy ?

in my motherboard bios I have two choices for operating system: uefi and legacy or CSM. Which one I should choose and what is the difference ?

:)

 

Legacy to UEFI

I'm debating moving to UEFI form my now legacy BIOS (Award Software Int.) the mother board is a GA-Z68XP-UD3 rev1.0 on the the following link it shows a UEFI BIOS U1J )UEFI BIOS)

GA-Z68XP-UD3 (rev. 1.0) | Motherboard - GIGABYTE Global

Currently I'm running the legacy F10 BIOS. Does installing the UEFI BIOS automatically take care of everything to get to the UEFI BIOS? Installing the UEFI BIOS following the instructions using the Q-flash method GIGABYTE How to Reflash Motherboard BIOS

What's steps do I take and in what order?

Much appreciated

 

UEFI or Legacy?


So i went for a clean Windows 10 installation on my Aspire E13.
To boot from USB i had to switch from UEFI to Legacy, otherwise the USB drive would not be listed.

However. After the installation was finished my touchpad was not even recognized, i mean, i was not even listed in the device manager, so i read somewhere on the Acer forum that switching back to UEFI would solve this problem.
Well, i switched back to UEFI and now when the laptop start i recieve a message that says -No bootable device found!

Even if i try to use my bootable USB drive, the list is empty.
So i switched back to Legace again and guess what. Windows 10 starts like there was no problem.

So, what's the different between UEFI and Legacy? The bios tells me -Use UEFI on Win 8 and above to use all the benefits.
And why oh why can't i boot up from a existing and working Windows 10 drive using UEFI?

 

Quick UEFI/Legacy question

I think it used to say, BIOS mode UEFI, but could it be on legacy because my sapphire hd 7970 only have legacy, gigabyte hd 7970 have uefi?