It is available on the Debian website at will install upgrade files in place, even on a running system.Sometimes, a revised package will require the installation of a newly revised version of another package, in which case the installation will fail until/unless the other package is installed.The second reason, which is related to the first, is that the software you are running on your device most certainly contains bugs.
It uses the geographic location of the user and other information to choose the best mirror that can serve the files.
In case there's some important note to add to the upgrade process, the packages will alert the user, and often provide a solution to a possible problem.
You should also read the Release Notes document that describes the details of specific upgrades.
(Wheezy, ZSH, aptitude 0.6.8.2) Update : With the new apt you can do now : will list available updates without actually installing.
First command updates package index files before simulated (thus -s) upgrade is done.