WinPE is packaged with a large collection of drivers, however, there are many devices that are not part of the WinPE list of drivers. For WinPE to recognize your device, you need to add a driver. Your device then communicates with WinPE and loads at boot time of WinPE.
The main devices to be concerned with are hard drive/RAID controllers, network interface cards (NIC) or USB controllers and USB hubs.
If you intend to use a network device then you need to ensure that your NIC is supported in WInPE. If your hard drives are attached to a RAID controller then you need to ensure that your RAID Controller is supported in WinPE, either by a WinPE driver or by adding a driver.
A device driver is a collection of files (also referred to as a driver package) and generally comprises of:
Your best sources for driver packages are the CD that comes with your device (or motherboard) or downloads from the vendor website as a ZIP file. When downloading a package from the vendor website, it is best to source a ZIP package for easy extraction.