apt-get update

ahiliation:~$sudo apt-get update
[sudo] password for jeffrin:
Get:1 http://dl.google.com stable Release.gpg [189B]
Ign http://dl.google.com stable/main Translation-en_IN
Hit http://http.us.debian.org sid Release.gpg
Get:2 http://dl.google.com stable Release [2,540B]
Ign http://dl.google.com stable/main Packages/DiffIndex
Get:3 http://dl.google.com stable/main Packages [871B]
Ign http://http.us.debian.org sid/main Translation-en_IN
Ign http://http.us.debian.org sid/contrib Translation-en_IN
Hit http://http.us.debian.org sid Release
Hit http://http.us.debian.org sid/main Packages/DiffIndex
Hit http://http.us.debian.org sid/contrib Packages/DiffIndex
Hit http://http.us.debian.org sid/main Sources/DiffIndex
Hit http://http.us.debian.org sid/contrib Sources/DiffIndex
Fetched 3,600B in 5s (630B/s)
Reading package lists… Done
ahiliation:~$

update is used to resynchronize the package index files from their sources

http://www.oreillynet.com/cs/user/view/cs_msg/59408