The word orthodox, from Gk. orthodoxos "having the right opinion," from orthos ("right, true, straight") + doxa ("opinion, praise", related to dexein, "thinking"), is typically used to mean adhering to the True Faith.
Orthodoxy is opposed to heterodoxy ("other teaching"), heresy and schism. People who deviate from orthodoxy by professing a doctrine considered to be false are most often called heretics, while those who deviate from orthodoxy by removing themselves from the perceived body of believers are called schismatics.
To find out more about what Orthodox Christians believe click HERE
Also, an excellent resource about Orthodoxy is found HERE