Hi Susana,
Welcome to this Microsoft Project newsgroup
Duration is a measure of the working time elapsed between the start and
finish of the task, eg 10 days, whereas Work is the number of man-hours
required to be done within that time eg 20 man-hours, which could be shared
by one or many resources. The relationship is given by the essential
equation Units of resource X Duration = Work, ie Men*Hours = man-hours.
You might like to have a look at my series on Microsoft Project in the
TechTrax ezine, particularly #5-Working With Resources, at this site:
http://tinyurl.com/2xbhc or
this:
http://pubs.logicalexpressions.com/Pub0009/LPMFrame.asp?CMD=ArticleSearch&AUTH=23
(Perhaps you'd care to rate the article before leaving the site,

Thanks.)
FAQs, companion products and other useful Project information can be seen at
this web address: <
http://www.mvps.org/project/>
Hope this helps - please let us know how you get on
Mike Glen
MS Project MVP