Here at the east end is the altar.
The iconostasis holds the main icons and mark the boundary between the congregation and the altar. While the classic iconostasis holds 4 icons (from left to right, patron saint of the church, virgin Mary, Christ pantocrator and John the Baptist) the key are these two book-ending Christ's earthly life (from birth to 2nd coming). In larger churches many rows of secondary icons can be added.
The actual area of the altar is entered only by clergy during the service. For part of the service this holiest of areas is hidden behind shut doors.
Like Roman Catholics, the Orthodox place great emphasis on Mary, calling her the 'mother of God.'
Above Mary is a depiction of the descent of the Holy Spirit at Pentecost.