DHCP (Dynamic Host Configuration Protocol) - protocol used by client to obtain automatically an IP address from reserved address range and also some useful information about local network (DNS-server, search domain, default gateway).
Typically the obtained address is valid for a limited period and can be later reused for another client.
Sometimes it is useful to give same IP-address every time client connect to network. In this case server can recognize client using hardware address of network adaptor (MAC address). You can add a such clients to the table with Static addresses.
DNS (Domain Name System) - protocol to establish a relationship between human-friendly symbolic names and IP addresses.
In some distributions you can find a DNS-server in pair with DHCP-server. This server works automatically without any additional configuration and supports DNS-zone with name equal to the server domain name (see module "Domain"). This server will work only with statically (not DHCP or Zeroconf) configured network interfaces.
In this case:
Attention: DNS-server's database will be reset if you change domain name or change some static IP-address. List of static DHCP addresses will also be removed in this case.