Squid Non-Cacheable Object Question

[atit jariwala]

Hi all,
 I am using Squid 2.5 Stable2.

There are some objects that are negatively cached in Squid. What does it
mean to Negatively Cache Object?
What are the headers in Squid which cause Objects to be marked as
Non-Cacheable?

Regards,
Atit Jariwala