In IPv4 networks the neighbour tables are written with usage of the Address Resolution Protocol. Those tables are commonly known as "ARP-tables". They resolve IP-addresses (addresses in the network layer) in MAC-addresses (addresses in the link-layer) and vice-vesa. You can list the entries of this table by the command arp -a or ip neigh show.
On the other side, in the IPv6 internet protocol suite, the functionality of the ARP protocol is provided by a more advanced protocol named Neighbour Discovery Protocol.
The ip ntable show command provides information about the neighbour table of the given network device, therefore:
arp_cache stands for an ARP-table (ARP Cache) of an IPv4 network.
ndisc_cache stands for an NDP-table (Neighbor Cache) of an IPv6 network.