There is a nice page on the wiki about this introductory stuff and particularly about the use of the MVC acronym in Cat.
Personally I agree with nothingmuch at #catalyst irc channel:
in reply to Catalyst : Observations After Week 1
model should be the most complex reusable
thing you can do without wasting time
For me it means that business logic should go into the model - but there are people that use some other definition of business logic. Generally it is usefull to reuse the model class in cron job or some other administrative scripts.