SIDN, the organization which runs the DNS for the
.NL (Netherlands) top-level domain,
has produced a DNS/DNSSEC training course which explains very well what DNS and DNSSEC are
and how they work.
I highly recommend the training, and you'll find it at www.dnsseccourse.nl. (Flash required.) You can test acquired knowledge at the end of the training with a short quiz, but read the questions very carefully .... ;-)