Oracle DB package versus procedure

What is the difference between a package and a procedure? When should I use packages?

Here a series of great examples:
http://www.java2s.com/Tutorial/Oracle/0540__Function-Procedure-Packages/Packages.htm


Here the official documentation
http://docs.oracle.com/cd/B12037_01/appdev.101/b10795/adfns_pc.htm


Here a chapter on the advantages of packages
http://docs.oracle.com/cd/B10501_01/appdev.920/a96624/09_packs.htm