Sepia, what is it, really?

Leonardo da Vinci used sepia ink, according to wikipedia

I really didn't know what sepia is, except that old photos are often brown. I assumed that was from age. The color "sepia" is the red-brown color of cuttlefish ink. The word "sepia" comes from the Greek word for cuttlefish. So what does this have to do with photographs?













And do I color correct an 1877 photo to black and white? or leave it be? or something in between?

References

• "Old Sepia Photo Restoration Techniques" photoshop tips.
• Premiere Pro help