For signed applets, check out
What is a digital certificate? at http://www.esus.com/javaindex/j2se/jdk1.2/javasecurity/cryptdigcerwhat.html
For trusted applets check out
What is a digital signature? at
http://www.esus.com/javaindex/j2se/jdk1.2/javasecurity/cryptdigsigwhat.html
Cheers,
Joris