Microsoft Publisher is a bit like a more graphical version of Word. It's for creating stuff like letterheads, adverts, brochures etc. I think it's pretty rubbish really. It's quite a powerful program, but the templates are pretty lame and cheesy looking. I wouldn't put much time into learning it. It's pretty self explanatory anyway.
MS Publisher is pants... it's not a serious DTP program by any stretch of the imagination and as mikeyp says it's just Word with a few added bells and whistles. And for the record, as a graphic designer, I know of no printing company that can print from it.
For an english project I did in year 9, we were supposed to make a newspaper front page, I found publisher particularly useful then, especially as none of the other students or even teachers knew how to use it so were impressed with mine :)