Unlike most western languages, Japanese has an extensive grammatical system to express politeness and formality.
Most relationships are not equal in Japanese society. The differences in social position are determined by a variety of factors including job, age, experience, or even psychological state (e.g., a person asking a favour tends to do so politely). The person in the lower position is expected to use a polite form of speech, whereas the other might use a more plain form. Strangers will also speak to each other politely. Japanese children rarely use polite speech until they are teens, at which point they are expected to begin speaking in a more adult manner. See uchi-soto.
Whereas teineigo (丁寧語) (polite language) is commonly an inflectional system, sonkeigo (尊敬語) (respectful language) and kenjōgo (謙譲語) (humble language) often employ many special honorific and humble alternate verbs: iku "go" becomes ikimasu in polite form, but is replaced by irassharu in honorific speech and ukagau or mairu in humble speech.
The difference between honorific and humble speech is particularly pronounced in the Japanese language. Humble language is used to talk about oneself or one's own group (company, family) whilst honorific language is mostly used when describing the interlocutor and his/her group. For example, the -san suffix ("Mr" "Mrs." or "Miss") is an example of honorific language. It is not used to talk about oneself or when talking about someone from one's company to an external person, since the company is the speaker's "group". When speaking directly to one's superior in one's company or when speaking with other employees within one's company about a superior, a Japanese person will use vocabulary and inflections of the honorific register to refer to the in-group superior and his or her speech and actions. When speaking to a person from another company (i.e., a member of an out-group), however, a Japanese person will use the plain or the humble register to refer to the speech and actions of his or her own in-group superiors. In short, the register used in Japanese to refer to the person, speech, or actions of any particular individual varies depending on the relationship (either in-group or out-group) between the speaker and listener, as well as depending on the relative status of the speaker, listener, and third-person referents. For this reason, the Japanese system for explicit indication of social register is known as a system of "relative honorifics." This stands in stark contrast to the Korean system of "absolute honorifics," in which the same register is used to refer to a particular individual (e.g. one's father, one's company president, etc.) in any context regardless of the relationship between the speaker and interlocutor. Thus, polite Korean speech can sound very presumptuous when translated verbatim into Japanese, as in Korean it is acceptable and normal to say things like "Our Mr. Company-President..." when communicating with a member of an out-group, which would be very inappropriate in a Japanese social context.
Most nouns in the Japanese language may be made polite by the addition of o- or go- as a prefix. o- is generally used for words of native Japanese origin, whereas go- is affixed to words of Chinese derivation. In some cases, the prefix has become a fixed part of the word, and is included even in regular speech, such as gohan 'cooked rice; meal.' Such a construction often indicates deference to either the item's owner or to the object itself. For example, the word tomodachi 'friend,' would become o-tomodachi when referring to the friend of someone of higher status (though mothers often use this form to refer to their children's friends). On the other hand, a polite speaker may sometimes refer to mizu 'water' as o-mizu in order to show politeness.
Most Japanese people employ politeness to indicate a lack of familiarity. That is, they use polite forms for new acquaintances, but if a relationship becomes more intimate, they no longer use them. This occurs regardless of age, social class, or gender.
source : wikipedia