Do they just create site navigations? Or do they have do to more? If so, what are the most important tasks an information architect has to do?
What Information Architecture is and what an Information Architect does/did are different.
Information Architecture is the creation of navigational and meta data structures that affect the end experience, as mentioned in the previous answer.
In reality an IA deals not only with information but also functionality. In my past role as an Information Architect I did all the tasks I now do under the title User Experience Architect or User Experience Designer.
In short an IA will do more than just create site structures. There are still IA tasks that a good UX person will handle and a knowledge of Information Architecture is vital unless you are a interaction/visual designer who is just working under the banner of UX. Any UX person working beyond microsites should, in my view, be a capable IA.
For example if you've no knowledge of faceted navigation, controlled vocabulary and then it's worth reading up more on IA.
The recommended book on the subject is the classic 'polar bear' book. Information Architecture for the World Wide Web: Designing Large-Scale Web Sites
Information architects' tasks are:
Optional, migrated from content-editor role
Frequently in a team there is not separate information architect's position, so this role is taken by UX designer.