In several Spanish documents from after the Conquest, there are references to a 'secret language' spoken by the Inca nobility that most people could not understand. It suits the Quechua Academy in Cuzco to claim that this was some more original or perfect form of the language, they claim that they alone speak the perfect, pure Quechua and are therefore in a unique position to decree standards for the language.
In truth, however, there is no single Quechua language--instead there is what linguists called a dialect chain across most of Western South America, in which speakers of one Quechua language can understand the languages spoken by their immediate neighbors, but not a language further from them.
Quechua is divided into a number of dialects. The major division is into 'Central' and 'Peripheral' Quechua. These are not normally mutually intelligible. Mutual intelligibility is tricky, however (see the linguistics FAQ on'languages vs. dialects'). Speakers of the modern Quechua dialects don't interrelate much (if they do much travelling, they know Spanish), and so don't have the habit of deciphering other dialects.
Quechua has been heavily influenced by Spanish, of course. Some 30% of the lexicon, even in monolingual speakers, comes from Spanish. Inorder to learn Quechua, it requires some vast changes of mind-set as learners try to master bipersonal conjugation, conjugation dependent on mental state and veracity of knowledge, spatial and temporal relationships, and numerous cultural factors.
Quechua has a three-vowel system: a, i, u. The vowels, like computer geeks, spread out to fit the space available-- an i, for instance, may be heard as [i], [I], or [e]. Bilingual Quechua speakers may approximate the five-vowel Spanish system (but they often get it wrong, producing a distinctive Quechua accent called motosidad).
The Incan language was based on nature. All of the elements of which they depended, and even some they didn't were give a divine character. They believed that all deities were created by an ever-lasting, invisible, and all-powerful god named Wiraqocha, or Sun god. The King Incan was seen as Sapan Intiq Churin, or the Only Son of the Sun.
Quechua was very distinctive language. Inca were very accurate in their language. The only imperfection that the Inca language had was that it did not have a written language. This would have really affected many of the Inca citizens. Quechua was under a certain procedure of only oral communication. The oral communication was mainly to maintain their culture.
Inca traditions survive stubbornly in Peru today. In remote villages older people speak only Quechua, the Inca language. They also keep Inca customs in food, music, and religion that have all but died out in urban areas.