Package management, involuntary upgrade

[Ameurlain Antoine <aameurlain@prosodie.com>]

Hi,
  I would like to know if there's a simple start document about 
debian package management (e.g. an howto). dpkg, apt-get, dselect
are mighty but quite hard to really understand. 
I would also like to have a description of the sort of link there
are beetween package, an also a description of package states :)

The fact is that i recently commited an error on my debian 2.2: 
upgraded libc6, libc6-dev, libstdc++, libg++, libgcc, lib cpp. 
Sample output:

Preparing to replace libc6 2.1.3-10 (using .../libc6_2.2.1-1_i386.deb) ...
Unpacking replacement libc6 ...
Replacing files in old package ldso ...

I didn't want that, but ... it happened. Now i'm quite nervous 
about my running http (compiled one), postmaster (deb package) etc.
Are there reasons for me to be nervous ?
Should I re-install my old packages ? How to "downgrade" packages ?

Thank you in advance