Look in the BOL why these databases are important, just to give a small
hints of the features of the database:

-master (like the names tells its the mater which contains every
information like server users etc. It can be rebuilöding once the
system is crashed, but restoring this is a much easier job to do)
-msdb (stores all your agent job)
-model (is used a a template for other databases when they are created,
if you defined any templates in your company they can be pulled out of
this one, if you don´t you don´t have to bother)

HTH, Jens Suessmeyer.