DHCP, Domain Host Configuration Protocol, is server-client service, which provides managed configuration of all necessary parameters for full computer network life (i.e. address, net mask, DNS server address, etc.).
DHCPv6 process operates in both stateless or stateful mode. In stateful mode, address and other network parameters, such as NTP or DNS server address, are provided to clients. DHCPv6 node knows all assigned addresses and it is able to operate on per client (DUID) basis.
Stateless mode is used in situation where addresses are provided by stateless service (SLAAC) and DHCPv6 service is used to provide additional parameters, which cannot be delivered by stateless service. There is a special flag in Router Advertisement (RA) messages called "O" - other stateful configuration. This flag instructs clients to query for DHCPv6. If no RDNSS option is provided in RA then client may check for DHCPv6 regardless of whether the O flag is set or not (see RFC4339).