Is there a difference to having a site such as http://mrobledo.dmldc.net being part of the main domain or subdomain in terms of how a search engine will search for it? And why can it also have the www in front like in http://www.mrobledo.dmldc.net Does these change how it can be accessed. Thanks

It would depend on the search engine on how much sub1.domain.com would affect it versus domain.com

Now as for sub2.domain and www.sub2.domain.com. They could be the same thing, though technically they could also go 2 different places, depends on how you configure it. The www really isn't necessary for accessing a site, but has been common place just out of habit I guess. www if you think about it is like another kind of subdomain.