Heliopolis is a sprawling residential and commercial area in the north-east of Cairo. Known locally as Masr al-Gadida ("New Cairo"), it sort of merges in with other suburbs, such as Nasr City to the south, and al-Zeitun to the north-west.
To confuse matters somewhat, there is also an area of Cairo known in English as New Cairo, which is a way south-east of Heliopolis, and roughly east of Maadi. In Arabic, this area is usually known as al-Qahirah al-Gadida, though occasionally people will refer to it as Katameya, or Tagamuaa!
In Pharaonic times, just north-west of modern day Heliopolis was an important religious site, known by the ancient Egyptians as On. It was the Greeks that named it Heliopolis, the "City of the Sun"; hence the modern name of the neighbourhood. Whereas Memphis was the administrative capital of the Old Kingdom, On was its religious capital, and source of the most important ancient Egyptian creation myths.
During the New Kingdom, the importance of On declined, and in 525 BC the Persians destroyed it. Although it was rebuilt and enjoyed a brief resurgence, the rise of Alexandria as a centre of learning spelled the death knell for On (known by then as Heliopolis). By Roman times, it might as well have never existed. Nowadays, nothing really remains of this history, although there is a tree in the area that is said to have sheltered the Holy Family during their flight through Egypt.
The modern suburb of Heliopolis was developed in the late 19th Century as a response to Cairo's burgeoning population. It was designed to be a self contained, leafy city for wealthy Egyptians and foreigners, and linked to Cairo by trams. In the 1950's, the foreigners all left, more middle class Egyptians moved in, and poorer areas also started to spring up. Heliopolis was swallowed up by Cairo proper.
Nowadays, Heliopolis is known mainly for its restaurants and nightlife, and for the occasional Art Deco building amongst the general concrete sprawl. It also boasts the City Stars shopping centre (technically, in Nasr City), a truly modern mega-mall.
It's quite common for business travellers to stay in Heliopolis, since it is near the airport, out of the hustle and bustle of the main city, and contains a number of decent hotels, such as Le Meridien Heliopolis, the Sonesta Hotel, and the Sheraton Heliopolis.
In terms of sights for tourists, the main attraction is probably the October War Panorama in Nasr City, which will almost have you convinced that Egypt won the 1973 war with Israel! There are also a number of theme parks aimed at kids in Heliopolis, such as Sindbad, and – if you head out into New Cairo – there are golf courses such as Katameya Heights Golf and Tennis Resort. Nasr City is also home to the Cairo Stadium, for if your visit happens to coincide with an important football match!