'Orthodox' Judaism comprises many different practices and viewpoints, under the umbrella of certain commonly held beliefs. It os 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.
Practices of Orthodox Judaism
These are commonly held to be:
Shabbat observance, including the level and stringency of observance, Kashrut observance, including the same, Levels of observance of Tznius ('modesty'), including dress sense, style of clothing worn, eg tight skirts or skirts with slits in them, maybe acceptable to certain Orthodox women but considered unacceptable to others.
Differences between Orthodox and Ultra-Orthodox sects
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.
Halachic definiton of an Orthodox Jew
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.