In speaking and writing alike, Japanese expects the speaker or author to pay proper respect to the audience and to the individuals who might be referred to. There is also a certain formality expected in formal situations, and this compounds the emphasis on showing respect through language. While not unique to Japanese, the language used to show this respect is called keigo.
Even the simple act of someones going somewhere is expressed differently depending upon who is going where. If you yourself are going to see a higher-ranking person, the verbs for “go” are mairu and ukagau. (Because this is used to indicate your own lower status, they might be called humblifics as opposed to honorifics.) Should a higher-ranking person go somewhere, the honorific verbs would be irassharu and oideninaru.
There are also levels of formality. If you are talking with close friends, “go” would easily be a casual iku with the masculine yo or the feminine wa markers added on. In a more formal situation, however, even among equals, it would be ikimasu or ikundesu. In effect, the verbs in formal situations take the more polite –desu or –masu endings, making these what are called “desu / masu” situations.
These same principles also apply to things and even attributes. A friend might be kirei (pretty), but the boss’s daughter is okirei (pretty with the honorific “o” prefix). Likewise, you send a tegami (letter) but your professor sends an o-tegami. People in elevated positions have go-shokugyou (occupations), –kangae (thoughts), and go-kazoku (families, with “go” being an alternative reading for the same honorific kanji.)
Keigo is usually understood to mean these three (honorifics, humblifics, and language), but there are also other ways to show respect for other people or places. For instance, rice (and by extension, food) can be referred as meshi or gohan, the stomach as hara or onaka, and more the just-us-guys expression first and the mixed-company expression second in both examples.
By the same token, using Chinese-origin terms is seen as more formal than indigenous Japanese terms, examples being hajimeru vs kaishisuru (both meaning to “start”), hare vs. seiten (clear skies), and sugu vs. sassoku (soon). It is not only words that have different levels of formality and respect. The same thing happens to entire phrases. For instance, moving from informal to formal / deferential, “I’m sorry” can be gomen, gomen-nasai, or moushiwake-gozaimasen; “yes” hai, sou desu, and sayoude-gozaimasu; “I understand” wakatta, kashikomarimashita, and shouchi itashimashita.