If I had to identify a distinguishing feature of Indonesian food from other Southeast Asian cuisines, it would be the prominence of peanuts and peanut sauces in many dishes, and a slight tip of the balance of spiciness in favour of sweetness, (often accomplished with coconut or coconut milk) -- generally much milder, subtler flavouring than in Thai food especially.
Many Indonesians prefer to eat with their right hand, balling up rice and using it to scoop from other communal dishes like curried chicken or vegetables. Fish is a wonderfully tactile experience eaten with the hands, using gentle massaging with your fingers to 'fish out' the bones, and smaller fish are eaten whole, bones, heads and all. As in many Southeast Asian cuisines, a balance of texture features large in Indonesian meals, and the crunchiness of those little crispy fried fish with a small ball of rice softened and dripping with curry sauce was a delight that I indulged in often while travelling in Indonesia.
Food stalls are everywhere in the streets, markets and festivals of Southeast Asian countries, but my recollection is that they were fewer and less varied in Indonesia than in the street food culture of Thailand, Malaysia and Singapore. In Indonesia, the warung might be a roadside shack with an eating counter or a small restaurant.
If you're planning to travel in Indonesia, understanding a few Indonesian Food Terms will help you decipher a menu or ask for a snack or dish at a street market or warung. If food is a highlight in your travels, have a look at the World Food Companion for some inspiration and some practical tips that have helped me stay healthy and well-fed over many years of travel.
If you're going to try your hand at cooking Indonesian, the Recipe for Travel's Indonesian Recipes Index has a few quick and easy recipes to get you started. Once you've mastered a good peanut sauce, you'll be able to create a great selection of popular and less familiar Indonesian dishes. A cookbook with good coverage of all Southeast Asian cuisines and a glossary of regional ingredients might also be worth an investment if you really fall in love with the food of Indonesia, Thailand, Malaysia and Singapore.