'Orthodox' Judaism comprises many different practices and viewpoints, under the umbrella of certain commonly held beliefs. It is therefore not 'one' main sect of Judaism, but many.
It is different to Ultra-Orthodox Judaism ('Charedim'), in that while certain practices and beliefs are held in common between the two, these are often more stringently applied and kept by Charedim.
Certain practices distinguish 'Orthodox' Jews from their secular or cultural counterparts.
These are commonly held to be:
While observing the laws of tznius, many Charedim consider Orthodox/ modern Orthodox observance of this as not stringent enough, for instance married Orthodox women may cover their hair with either a hat, partially, fully or not at all, as they see appropriate, and not a sheitl. Even in the Charedi community, there is a debate as to whether or not sheitls are truly 'tznius', as in some cases the sheitls are more appealing than the wearer's actual hair underneath.
Halachically, an Orthodox Jew is any child born of a Jewish mother. For males, there is an additional requirement to be circumcised at 8 days old, as a way of entering the Biblical covenant, as told to Abraham in Genesis.
While the father need not be Jewish in order to have a fully Jewish child, it is preferable for cultural concerns, and for ensuring the continuation of Judaism and Jewish ideals.
Even if a child is born Orthodox, an Orthodox family by this definition need not be a fully practising Orthodox Jewish household, and may be more culturally-Jewish in their daily lives.