We use Debian here at our shop, so here are some basic apt-get commands.
Lets look up some policy info for a package we want to install.
apt-cache search nfs
nfs-common - NFS support files common to client and server nfs-kernel-server - support for NFS kernel server
apt-cache policy nfs-common nfs-common: Installed: (none) Candidate: 1:1.2.2-4squeeze2 Version table: 1:1.2.2-4squeeze2 0 500 http://mirrors.work.com/debian/ squeeze/main amd64 Packages 100 /var/lib/dpkg/status