> Does anyone could give-me  some explanation about the diferences between
Eclipse e NetBeans.

I'll give it a go, but I haven't used either of them for some time.

> Are they the most important  IDE's to java development?

No, but I bet they like to think they are.

> Which the main  features?

I Think Netbeans is it is a fully featured IDE, meaning it has every
major feature from every other IDE. It doesn't have the best
implementation of them all though.

Eclipse ... Well I don't like eclipse  but lots of people do. As far as I
can tell standalone it is more like a editor than a IDE, but you can
plug stuff into it and get a very IDE like experiance.

I could be wrong, and just got a retarded point of view from it.

> Are they used to build  desktop and web applications?

Sure, why not. After all you build the applications  with Java, not a IDE.

> Are there versions  to Win32 and Linux?

Yes.

> Are they open  source projects?

I think so, or if Netbeans isn't it is in that near Open sourced level
that Sun so loves.