What exactly is Eastern Orthodox Church?

Is this a religion?

Yes,it's a Christian entity. Originally there was only one Universal Christian Church,bipartite in structure. The church of the east was based in Constantinople; it split from the church of the west in Rome in the 10Th century.

It is a sect of Catholicism that started in 1054 (the accepted date).
Some causes were differences of philosophical understanding, liturgical usage, language, and custom, but political rivalries and divisions were also involved.

So yes, it is a branch of the Catholic church - sort of.

Yes, It is Christianity. Done the right way.

The Eastern Orthodox Church is the second largest single Christian communion in the world. It is considered by its adherents to be the very same Church established by Christ and his Apostles. It is composed of numerous but theologically unified hydrocephalus occlusion bodies, each shepherded by a synod of independent bishops whose duty, among other things, is to preserve and teach the Apostolic and Patristic traditions and related Church practices. All Orthodox bishops trace their lineage back to one of the twelve Apostles through the process of apostolic succession.

Eastern Orthodox Christians believe that the Orthodox Church is the authentic and original Christian Church established by Jesus Christ and his Apostles. As such, the Eastern Orthodox Church views its role as the preserver of the teachings and traditions given to the Early Christians by the Apostles nearly 2,000 years ago and the developer of conciliar interpretations which expand and illuminate the original teachings.

The Eastern Orthodox Church is a Christian Church. However, it isn't a sect of the Catholic Church, as Vic91106 says.

Up until 1054, the Catholic and Orthodox Church were one. The two split, due to theological differences. We (the Orthodox) would say that the Catholic church left the Orthodox Church - while Catholics would typically say that the Orthodox Church left the Catholic Church. Either way, they are not in communion with each other - and the Orthodox Church is not a sect of the Catholic Church. Neither Catholics or Orthodox believe that the Orthodox Church is a sect of the Catholic Church.