'apt-get update' goes out to the servers listed in /etc/apt/sources.list
and downloads an updated list of all the packages available at those
sites. 

Let's say a new version of openssh is released with a bug fix. You read
the advisory up and you do an 'apt-get install ssh'.  You'll get a
message back saying that you have the latest version already installed 
b/c your package list has not been updated. You first have to do
an 'apt-get update' to update the list, and then 'apt-get install' to
download the package. I'm pretty new to Debian, but I believe a reason
for this is so that apt-get doesn't have to scour the net for packages
everytime you install something.  Most people have their systems setup
with a cron to 'apt-get update' every night so that they have the latest
package list. That way when they need to install something, apt-get knows
exactly where to go.

/me waits for der.hans to correct his mistakes :)

~Deepak


On Sep 19 2003, at 13:36, Michael Havens was caught saying:
> What does this download? I would think that it would download a file with a 
> list of files and their version number. Right? Then when you run apt-get 
> upgrade it would compare that list with a list of files on your system and 
> download those files that differ. BUT NO!  I run apt-get update and it 
> downloads a bunch of files that I am not sure what they contain. And then 
> apt-get upgrade downloads the files again (because I don't know what the 
> first file it downloaded was). So what exactly does apt-get update give you?
> -- 
> <:-)~MIKE~(-:>
> 
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-- 
Deepak Saxena - dsaxena@plexity.net - http://www.plexity.net