Japanese (日本語) is a language spoken mainly in Japan and in Japanese communities abroad.
The written language consists of three kinds of characters: kanji, hiragana, and katakana. Japanese can be written "western" style (horizontally left to right, then top to bottom) or in traditional Japanese style (vertically top to bottom, right to left). Kanji consists of thousands of sometimes complex written characters which were adopted from the Chinese language. Their meanings were altered over time, so the same kanji can have a completely different meaning and pronunciation in Japanese and Chinese. Hiragana is a phonetic system of 46 characters developed in the 9th century, and is exclusively Japanese. Katakana is a separate phonetic system consisting of 46 characters, and is used mainly for words "borrowed" from other languages. Hiragana and katakana together are often referred to as kana.
Spoken Japanese includes fewer sounds than English - thus, Japanese pronunciation is generally easier for English speakers to learn than, say, German (which contains sounds not in English spoken speech). There are many regional dialects of Japanese that are distinct to native Japanese or to the trained ear. For example, the Osaka accent is markedly different from the Tokyo accent.
Spoken Japanese relies heavily on context. The subjects of sentences are often left out of sentences entirely, as they are assumed to be understood by the context of the conversation. Plurals and singulars are basically the same ("I have a dog" or "I have dogs" is "犬を飼う"), as are present and future tenses of verbs ("I go" or "I will go" is "行く").
For foreigner speakers, the toughest thing to learn is often the honorific system, or keigo. In Japanese, you use a different sentence and verb structure depending on who you are talking to and whether they are your social superiors, inferiors, or equals - or whether the person is a stranger, a child, or a close friend.
There are many resources available for people interested in learning to speak and/or write Japanese. Please see the following related articles if you are interested: