The simple answer to this question is that the Orthodox Church is the original Church established by Jesus Christ.
The Apostles were sent by Christ to preach the Gospel and build the Church. Individual parishes founded by the Apostles still stand today as Orthodox outposts throughout the world.
The Orthodox Church is the continuation of what was originally established in the beginning.
Most only know about the Roman Catholic Church and the various Protestant churches that split off from her.
The Orthodox Church is the original deal.
Check out this short video by Fr Andrew. It's less than 3 minutes and worth your time.
In an age when too many churches are chasing being relevant and following the winds of the culture, the Orthodox Church preserves the ancient deposit of the Christian faith.