'Schema Vs. XML' and 'Namespace Vs DTD'

Hello all,

Greetings.

I need a couple of clarification regarding the XML, listed below:

1. I am having an XML file and its Schema. My clarification is how to 
validate my xml file against the schema. Is there any tool/engine 
exiting? Please advice.
2. What is namespace in xml? and How to implement this in DTD? Please 
explain me with example.
Your reply will be appreciated, many thanks in advance.

Best regards
Arul