Old Delhi is a chaotic, noisy, filthy, fascinating place. It feels as if half of humanity is crowded into the narrow streets some of the time! The main street in Old Delhi is Chadni Chwok, the market street that runs west from the Red Fort. Off this street are endless tiny alley ways, courtyards, and streets, many of which are too narrow for anything other than pedestrain traffic (or cows.) Every kind of shop is here, from fabric and sari emporiums, to small shops selling heaped, fragrent spices. There are also a hundred places to get a cheap meal, drink, or snack, and many have the distinct advantage of cooking in front of you, so you can have a quick look at the cleanliness of the equipment and the cook's fingernails!
As well as being a great place to shop, the area is fascinating. There are people selling, chatting to their friends over a drink, playing cards - there are women chasing up their children for meals, and endless games of tag, chase the chicken, and dodge the rickshaw played by children. It's great to shop for a while, then buy a drink and sit and watch the world go by around you.
As with any busy area, it pays to keep an eye on your wallet and camera here - these streets have a bit of a record for pickpocketing.
Before the partition of India in 1947, Old Delhi was a mostly Muslim area, but in the massive population upheavels at that time, many left and went to Pakistan or East Pakistan (now Bangladesh), and were replaced by Hindus leaving other areas. Much of the architecture, especially the housing, is Islamic in appearance, with tall houses built around coutyards, and delicate lattice carving. Many of the buildings are in a seriously bad state of repair, but interesting nonetheless.
I really recommend you read the book City of Djinns, by
William Dalrymple, while you are in Delhi, or before you go. The writer, who weaves amazing portraits with his work, spent a year living in Delhi. The book is both a historic account, and modern description of the city, and Old Delhi is particuarly well-explored. It will make you see things in the city you would otherwise miss, and you will appreciate the capital so much more having read it.