The principal commercial products of Bihar are:

Crops - rice, wheat, lentils, maize (corn), sugar cane.

Fruits - mangoes, bananas, jack-fruit, and litchies.

Fibers - silk (particulary from the Bhagalpur region in the East, producers of a distinct quality of sillk, namely, tussar or tussah); and jute, transported to factories located mostly near Calcutta for easy export of the finished material.

Forest Products - hard wood timber, saal and sakhua from the north; also cane for weaving, particularly from the swamps in West Champaran district of North Bihar;  bidi leaves from the forests of the south (rolled with tobacco, it is the common smoke of the people); and lac - encrustation of the lac insect on the trees of the south from which shellac is made, and which is also the source of bangles.

Minerals - The Chhotanagpur plateau is indeed the single richest mineral belt in all of India; indeed, it is one of the richest mineral containing area in the whole world. It has frequently been called the Ruhr of India, after a similar mineral-rich area of that name (Ruhr) in Germany, which was a vital part of the German war industry during World War II.

Coal, iron ore, limestone (used in the manufacture of cement), mica, bauxite, copper are the prinicipal products.

North Bihar, a rich agricultural area, has many industries associated with agricultural products. There are numerous sugar factories scattered throughout the area. Many rice and edible oilmills also dot the landscape. It also has some sundry, but important, manufacturing plants, for example the Button Factory at Mehsi (East Champaran0, and the old and renowned rail wagon manufacturing plant, the Arthur Butler & Co, at Muzaffarpur. Immediately after independence however, a major industrial complex grew around Barauni. The industrial plants located there are: the Fertilizer Factory, the Oil (petroleum) Refinery Plant, and the Thermal Power Station. Recently, a Thermal Power Plant has also begun operation at Kanti, in the Muzaffarpur district along its border with East Champaran.

Regarding commerce and North Bihar, mention must be made of the gigantic annual cattle fair at Sonpur in the Saran district, close to the confluence of the Gandak and Ganges rivers. The fair is held around the religious festival of Kartik Purnima - full moon in the month of Kartik in the Hindu lunar calendar (corresponding to some time in Oct-Dec in the Gregorian calendar), which marks the end of the holy month of Kartik. Kartik Purnima in 1998 falls on Nov 4. This fair is reputed to be one of the world's largest such fair, where not just cattle but also exotic animals and horses and elephants are traded in large number. It attracts a large number of tourists from many countries. The Government of Bihar, through their Department of Tourism, provide many amenities for their boarding and lodging.  See details here in an advertisement in a Patna newspaper for the 1997 fair.

South Bihar bustles with heavy industries, boasting of two huge steel mills, The Tata Iron and Steel Co. in Jamshedpur and the Bokaro Steel Mills of the Steel Authority of India (formerly known as the Hindustan Steel Co.) at Bokaro (or Bokaro Steel City), a town formerly in the district of Dhanbad, but now a separate district. 

Ranchi is home to many industrial, mining  and commercial institutions. Among them are:

      Heavy Engineering Corporation (HEC), a public sector undertaking (PSU) of the Government of India. It comprises of three separate units, the Foundry Forge Plant, the Heavy Machine Building Plant, and the Heavy Machine Tools Plant. It was established in 1958 with Russian collaboration. Products include capital equipment, machine tools, and spare parts needed in core sector industries, for example, steel mills, power and atomic energy, coal mining, ship building, etc.

    Metallurgical and Engineering Consultants (India) Limited, another Government of India undertaking. It is one of very few companies in India which offers consultancy services in diverse areas - for example, software development, design and fabrication of blast furnaces, printing factories for bank notes, manufacturing of various kinds, etc. It has clients spread across all over India and in several parts of the world.They maintain offices in many countries.

    Research and Development wing of the Steel Authority of India, Ltd. (SAIL.) A massive organization of highly skilled engineers and scientists doing research in all aspects of steel mill operation. Its well-appointed administrative training center attracts trainees from many foreign countries - especially from the newly emerging nations of Africa which are engaged in the erection of steel   mills in their own country.

     If you are interested in numbers, then here is a recent quarterly report from the RBI (Reserve Bank of India, India's federal currency issuing bank), giving district-wise figures for amounts on deposit.

And here is an objective report about investment opportunities in Bihar. Thanks to Mukesh Pratap Singh (of Sitamrahi and Italy) fo this link.