(responding to Stefan)
> as there are no clear practices given in the books about
> scrum (or i have not found them) - can you tell me, what
> is the typical approuch to get the items of the product
> catalog. is it just making an interview with the potential
> users/product owner and then write down their "wishes"?
The two best resources to learn about this area, IMO, are
both books written by Mike Cohn:
"Agile Estimating and Planning" will probably be better at
getting you thinking the right way about Product Backlog.
"User Stories Applied" will probably be better at giving
practical advice on capturing and organising the requirements.
There's a third book I often recommend to the more traditional
or RUP oriented teams - "Requirements by Collaboration"
by Ellen Gottesdiener. There's a lot of useful stuff in it for
folk transitioning from traditional and needing to find better ways
of eliciting requirements from customers, but IMHO it stops a
bit too far short of full-on agile.