Massively oversimplifying:
The Domain Name System DNS translates names eg:
www.pprune.org into the underlying IP address(es) eg: 98.158.194.181 by looking up a database.
Now with any lookup the closer the server the faster the lookup. So if you have a fixed system like a desktop then generally your ISP 'should' provide a faster lookup.
BUT if you have a laptop which moves between networks then your ISP's server may not be accessible from another network - hence the value of using a public DNS server like Google.
As this is a critical part of web browsing you should use both Google servers eg 8.8.8.8 as primary and 8.8.4.4 as your secondary.
Needless to say as it is Google they are almost certainly hoovering up all the DNS queries, so if you are paranoid you may wish to use an alternative public DNS server.
HTH
EG