It is a tradition in the Church that Mary was so in close communion with God her entire life, so in-step with His will, that she never committed any voluntary sin (that is, willing herself to sin). Chances are, she did commit involuntary sins.
She was also in need of a Savior, just like the rest of us. She, like us, inherited a fallen human nature that was exposed to corruption and death, and she died involuntarily just like the rest of us (only Christ dies voluntarily), and therefore has to trust in Christ to raise her from the dead.
The Catholic dogma of the Immaculate Conception, however, is quite different. This argues that the Theotokos was prevented from receiving guilt of Adam's sin, and therefore did not inherit sin so that Christ could be born without sin. This is borne out of how the RCC understands Original Sin, believing that all humanity is actually guilty of Adam's sin, and so if Christ is borne human he would be guilty, too, and that can't happen. For the Orthodox, there is no "original guilt" and so there is no need for the Immaculate Conception doctrine.
On the contrary, many Orthodox argue that it removes the Virgin Mary from humanity by changing her nature, and therefore would give Christ a different nature than the rest of humanity. Extrapolating based on the Patristic quote, "what is not assumed is not saved"...if Christ's humanity is different than ours, we cannot be saved. Therefore, Christ must inherit our fallen nature, when he received from the Mother of God.