What's the diference between WDS and Root Access Point??
They're actually fairly different concepts. A Root access point is an infrastructure AP that associates clients, which is what almost every AP you install will be. WDS is a Cisco protocol that lets APs pass user credentials to each other to shorten the authentication process while roaming, among other things. A WDS AP is the AP that stores all the keys and manages them among APs.
Is that what you're looking for? Again, they're not really the same concept. Your WDS APs are usually root APs.