Question: I've been mandated to find out what [IBM Cognos](http://www-01.ibm.com/software/data/cognos/?pgel=ibmhzn&cm_re=masthead-_-products-_-sw-cognos) does, and I cannot find useful information on the subject apart what I can read from the IBM Website and [Wikipedia](http://en.wikipedia.org/wiki/Cognos).

What I'm after is some concrete examples of what Cognos can do for businesses and organisations that intend to use it.

Answer: 
A bit late, but for the benefit of anyone browsing ... Cognos BI is essentially web based reporting/ dashboarding/ analytics. Historically it connected to relational databases only; from v8.4 onwards (and moreso from v10) it also connects to OLAP cube data sources. It's designed for end user self service reporting and includes mobile as well as web connectivity.

Cognos FPM provides in-server memory OLAP cube modelling (based on the TM1 engine). A key point of difference is that it permits end-user writeback and is generally used for budgeting and 'what-if' scenario modelling. Modelling is facilitated by Rules, which enable data modification. It also scales to the max. As noted above it may be integrated to Cognos BI (as well as being stand-alone), which means that a single dashboard may include reports from both relational & OLAP sources, and provide planning. So it's very powerful.

Note that Cognos Express provides essentially the same tools for the midmarket.
A little late but in case anyone else comes here and is looking for information, I would like to enhance @paxdiablo's answer. He was talking only about Modeling and reporting tool which is the best known Cognos. 

There is also a powerful tool named Metric Studio which can track in an easy way, how business is performing. This tool is IMHO the best of the Cognos Suite, since it is truly BI for the high management.

Another thing that I love from Cognos (been using it since 2004) is the administration. From an IT perspective it is way easier to make things happen in Cognos rather than any other tool I've seen (BO included).

Just to name a few: you can link row-filtering with LDAP information (e.g. roles and customers); burst reporting through cognos content or email... the possibilities are huge.