Registrar in DNS is the organization that manages the reservation of domains which is publicly available. They should be accredited by TLDs like ".com" or ".org" etc or country wie TLDs like ".uk" or ".in". These registrars follows the common guidelines which helps for delegations.
DNS Master can be alternatively called as DNS Primary server which holds the Read Write copy of the zone data. This DNS Master will transfer the zone information to its secondary servers which holds Read Only copy of the data. Any modification for a DNS Zone can be done only on the Master DNS and this master will transfer it to the secondary servers via Zone Transfer.
Grid Master in Infoblox is a server which acts as a common place to access all the other servers in the member. This Grid Master can also run Protocol Services like DNS and DHCP. There is no difference in the way the Grid Master (if configured as DNS Master)talks to the secondary servers. However note that in Infoblox, the zone data between the primary and secondary DNS server can work either via "Zone Transfer" or via "Grid Replication" if both the Primary and Secondary servers are in the same grid.
Hope this helps!
