iReport is a GUI for JasperReports. It helps you lay out reports visually. You *may* be able to get away with using iReport only, but unless you know what it all means, it's unlikely to work well (and I always had to go back in and edit the JR file by hand, but I haven't used iReport for ~1-2 years, so it's probably better now).
I'd suggest you read a bit more thoroughly, though, because iReport has zero to do with web applications. It *designs* reports.
We're currently using JasperReports/iReports on our project and while it makes some things easier, other parts are slightly more complicated and it is difficult to get some good tutorials on using it.
You will need to know Jasper Reports in the form of how to use the .jrxml file (the jasper source file) or .jasper file (the compiled file) and how to pass in parameters etc as iReports only generates these files for you.
We wanted to pass javabeans into this rather than allowing JR to have a direct database connection and this again proved quite difficult and there was a lack of documentation on this, but maybe it is easier if you do. I believe that JR can also use hibernate settings etc if you use this.
Personally go for both if you are going to use JR, but while it's an IDE it doesn't make things the simplest (in my opinion).
Well it wasn't difficult once you knew how, I just thought there was very little documentation (perhaps also relying on google a bit more than I should have), but we used JR's JRBeanCollectionDataSource in the end, but like I said we got this is the end. It's not quite as big a deal as I made it seem in my last post. I think it was also to do with that when using iReports, to get test data for iReports to use, you needed to select a datasource, which after much reading up you could do using a static class to produce a List of test data objects. Again didn't think there was much helpful documentation on this.