It is possible to poison DNS by informing a server that a particular asset is at an IP address other than the one it really is located. There are a few ways of doing this, see here for an example.

Once the Server is poisoned then all requests to it will return the wrong IP for the asset that has been poisoned. The magnitude of the effect depends on the "level" of the DNS server. If I poison a server in my private domain then the only clients affected are my own. If I poison my ISP's server then all my ISP's customers are susceptible to the poisoning. If I can poison the root servers the entire internet would be poisoned.

The thread I started here is a beneficial example of DNS Poisoning.

[EDIT]

Fixed my Ooops on the second link....

[/EDIT]