Also, i had a look at Relax NG but didn't understand what it is. It
seems a alternative to Schema?

Yes.  And many people believe a superior alternative.  OpenOffice,
DocBook and TEI -- to name just a few high profile document schemas --
are all authored in RNG.

A replacement for DTD?

However it's not a W3C standard, or is it?

I like RNG because it's easy to learn (before learning it, I didn't
even know how to write a DTD), but is more powerful (for what I do)
than XML Schema.  Simple examples:

in RNG, you can condition validation on attributes; in XSD you cannot
in RNG you can validate irrespective of element order; in XSD you cannot

The other thing I like about RNG is Trang, which is a tool that allows
you to convert RNG schemas to XSD or DTD (assuming you don't use
RNG-only features, of course).