apt-cache policy shows installed and available, sometimes they are the same version if there is an updated package the versions will be different.
$ apt-cache policy iceweasel
iceweasel:
Installed: 10.0.4esr-2
Candidate: 10.0.4esr-2
Version table:
12.0-2 0
1
http://ftp.us.debian.org/debian/ experimental/main i386 Packages
*** 10.0.4esr-2 0
500
http://ftp.us.debian.org/debian/ unstable/main i386 Packages
100 /var/lib/dpkg/status
Read the apt-get man pages will explain in more detail. The nice thing about debian is the smart package management system. Install apt--listbugs and apt-listchanges, and enjoy the ride. Or the aptosid manual.