Need for an ODS(operational data store)

What is the need for an ODS(operational data store) in data warehousing?What are the advantages of using an ODS?

Incremental aggregation

How to use incremental aggregation in real time?
What is meant by source is changing incrementally?
Please explain with an example..

Is sorter active or passive?

Why is sorter an active transformation?
What happens if you uncheck the DISTINCT option in sorter?

Top down and bottom-up approach

Loading data from data-marts to datawarehouse or datawarehouse to data-marts which one is considered better and why?Which approach did you use in your project?

Differences between lookup and joiner?

What are the differences between lookup and joiner transformations?Which one is better performance-wise and why?

Differences between informatica 6.1 and 7.1

What are the differences between Informatica powercenter versions 6.1 and 7.1?
Is there any architectural difference between them?