Domain Name Servers (DNS) are the Internet's equivalent of a phone directory. The DNS translates domain names into Internet Protocol (IP) addresses. Names are easy for people to remember, but computers access websites based on IP addresses.
The DNS Cache is simply a local temporary database that has records of all recent visits and attempted visits to websites and other internet domains. This is helpful because instead of having to go out to the internet to find the IP address to the place you want to go, your local DNS Cache will already have that translated and ready to go immediately.