Not considered to be the most beautiful city in the Amazon by any stretch of the imagination, Manaus is nevertheless is an important stop on any Amazonian itinerary. It has important air and boat links to other places in the region, as well as to other countries due to its airport and boat terminal on the Rio Negro. You can reach both the mouth of the amazon (1500km away) and Leticia, Colombia (1000km away) where you can change boat to go to Iquitos, Peru.
The population is around 1.6 million and it's very busy and a little seedy at night especially around the port area. Muggings are common and it's advisable to take taxis after dark.
Most of the Manaus hostels lie in the Zona Franca area which is the area south of Avenue Sete de Septiembre. There are many hostels in the area, but seem a bit expensive for what you get, but most have wifi and some dormitories have air conditioning which can be a godsend when you get high temperatures and 100% humidity.
There are many restaurants all over Manaus. Many offer good value buffets or comida por kilo (pay by the weight of your plate) but vary in standard so always check the food out before committing. Buffets go for R$8 and up while comida por kilo is about R$15 per kilo and up.
Manaus has two famous landmarks; the fabulous Municipal Market near the port (it's sometimes closed for refurbishment, check before going) and the famous Theatre Amazonas, a beautiful whitewashed ornate building in the hear of the town. Free orchestral recitals are put on occasionally. Check with your Manaus hostel's information desk for details.