BNG---Broadband Network Gateway
BRAS---Broadband Remote Access Server
They are the same thing. Previously it was called BRAS, but now its called BNG.
BNG (Broadband Network Gateway) is the first node for a subscriber to access network service.
In simple english, when someone powers on his laptop, his laptop send info to BNG router (by PPPoE protocol), and BNG talks with other element in the internet provider company to check if this is their real client or not, paid money etc (RADIUS server).
If all ok, some other machine (DHCP server) gives BNG an IP address for this guy which BNG passes to him. Upon receving IP address, this guy now has the access to internet.