Your typical server log directory looks something like this:

access.log
access.log.1
access.log.1.gz
access.log.2.gz

Now what if you want to extract data from day x to day y? You could use gzip and grep to uncompress the files, but there's a better way: the z commands.

All you have to do is:

zgrep "2010" /var/log/apache2/access.log*

More info on the subject can be found here: http://www.thegeekstuff.com/2009/05/zcat-zless-zgrep-zdiff-zcmp-zmore-gzip-file-operations-on-the-compressed-files/