What would be the difference between a DNS on a webhost or other dns's i see on websites such as opendns.com , and DNSMADEEASY.COM


Why do they use these dns services instead of dns on there servers?

OpenDNS is a resolving name service.
DNSMadeEasy is an authoritative DNS service.

Wikipedia will explain it all:
http://en.wikipedia.org/wiki/Domain_Name_System

What makes them better than ones that come with domains?

Eh... Just use the DNS that comes with your registrar.
Unless you need something that you were told "you can not do with your current DNS system" just stay with the free stuff at your registrar.

Will the free one be able to handle high traffic?

If not... then you upgrade to a paid one. Can a paid one even handle the traffic? All good questions... no one really knows.
But unless your site is the size of Amazon by T1-81 calculator can probably handle the CPU load to take care of all of your DNS.