Katakana (片仮名、カタカナ, Japanese pronunciation: [katakaꜜna]) is a Japanese syllabary, one component of the Japanese writing system along with hiragana, kanji and in some cases the Latin script (known as rōmaji). Reference: https://en.wikipedia.org/wiki/Katakana
Excerpt from Wikipedia: The word katakana means "fragmentary kana", as the katakana characters are derived from components or fragments of more complex kanji. Katakana and hiragana are both kana systems. With one or two minor exceptions, each syllable (strictly mora) in the Japanese language is represented by one character or kana, in each system. Each kana represents either a vowel such as "a" (katakana ア); a consonant followed by a vowel such as "ka" (katakana カ); or "n" (katakana ン), a nasal sonorant which, depending on the context, sounds either like English m, n or ng (ŋ) or like the nasal vowels of Portuguese or Galician. In contrast to the hiragana syllabary, which is used for Japanese words not covered by kanji and for grammatical inflections, the katakana syllabary usage is quite similar to italics in English; specifically, it is used for transcription of foreign-language words into Japanese and the writing of loan words (collectively gairaigo); for emphasis; to represent onomatopoeia; for technical and scientific terms; and for names of plants, animals, minerals and often Japanese companies. Katakana are characterized by short, straight strokes and sharp corners. All katakana used in modern orthography are interspersed as vocabulary sets throughout the rest of the course. First come the vowels:
Another excerpt from Wikipedia: In modern Japanese, katakana is most often used for transcription of words from foreign languages or loanwords (other than words historically imported from Chinese), called gairaigo. For example, "television" is written テレビ (terebi). Similarly, katakana is usually used for country names, foreign places, and foreign personal names. For example, the United States is usually referred to as アメリカ Amerika, rather than in its ateji kanji spelling of 亜米利加 Amerika. Katakana are also used for onomatopoeia, words used to represent sounds – for example, ピンポン (pinpon), the "ding-dong" sound of a doorbell. Technical and scientific terms, such as the names of animal and plant species and minerals, are also commonly written in katakana. Homo sapiens, as a species, is written ヒト (hito), rather than its kanji 人. Katakana are often (but not always) used for transcription of Japanese company names. For example, Suzuki is written スズキ, and Toyota is written トヨタ. As these are common family names, Suzuki being the second most common in Japan, using katakana helps distinguish company names from surnames in writing. Katakana are commonly used on signs, advertisements, and hoardings (i.e., billboards), for example, ココ koko ("here"), ゴミ gomi ("trash"), or メガネ megane ("glasses"). Words the writer wishes to emphasize in a sentence are also sometimes written in katakana, mirroring the usage of italics in European languages. Pre-World War II official documents mix katakana and kanji in the same way that hiragana and kanji are mixed in modern Japanese texts, that is, katakana were used for okurigana and particles such as wa or o. Katakana was also used for telegrams in Japan before 1988, and for computer systems – before the introduction of multibyte characters – in the 1980s. Most computers of that era used katakana instead of kanji or hiragana for output. Although words borrowed from ancient Chinese are usually written in kanji, loanwords from modern Chinese dialects that are borrowed directly use katakana instead.
The very common Chinese loanword rāmen, written in katakana as ラーメン, is rarely written with its kanji (拉麺). There are rare instances where the opposite has occurred, with kanji forms created from words originally written in katakana. An example of this is コーヒー kōhī, ("coffee"), which can alternatively be written as 珈琲. This kanji usage is occasionally employed by coffee manufacturers or coffee shops for novelty.
Katakana are also sometimes used to indicate words being spoken in a foreign or otherwise unusual accent. For example, in a manga, the speech of a foreign character or a robot may be represented by コンニチワ konnichiwa ("hello") instead of the more typical hiragana こんにちは. Some Japanese personal names are written in katakana. This was more common in the past, hence elderly women often have katakana names. This was particularly common among women in the Meiji and Taishō periods, when many poor, illiterate parents were unwilling to pay a scholar to give their daughters names in kanji. Katakana is also used to denote the fact that a character is speaking a foreign language, and what is displayed in katakana is only the Japanese "translation" of their words.
Some frequently used words may also be written in katakana in dialogs to convey an informal, conversational tone. Some examples include マンガ ("manga"), アイツ aitsu ("that guy or girl; he/him; she/her"), バカ baka ("fool"), etc. Words with difficult-to-read kanji are sometimes written in katakana (hiragana is also used for this purpose). This phenomenon is often seen with medical terminology. For example, in the word 皮膚科 hifuka ("dermatology"), the second kanji, 膚, is considered difficult to read, and thus the word hifuka is commonly written 皮フ科 or ヒフ科, mixing kanji and katakana. Similarly, difficult-to-read kanji such as 癌 gan ("cancer") are often written in katakana or hiragana. Katakana is also used for traditional musical notations, as in the Tozan-ryū of shakuhachi, and in sankyoku ensembles with koto, shamisen and shakuhachi.
Some instructors teaching Japanese as a foreign language "introduce katakana after the students have learned to read and write sentences in hiragana without difficulty and know the rules." Most students who have learned hiragana "do not have great difficulty in memorizing" katakana as well. Other instructors introduce katakana first, because these are used with loanwords. This gives students a chance to practice reading and writing kana with meaningful words. This was the approach taken by the influential American linguistics scholar Eleanor Harz Jorden in Japanese: The Written Language (parallel to Japanese: The Spoken Language).
A Small Bit of History
Excerpt from Wikipedia: Katakana was developed in the 9th century (during the early Heian period) by Buddhist monks in Nara by taking parts of man'yōgana characters as a form of shorthand, hence this kana is so-called kata (片, "partial, fragmented"). For example, ka (カ) comes from the left side of ka (加, lit. "increase", but the original meaning is no longer applicable to kana).
Trick Questions and False Symmetries
There are many symmetries in the kana presented above. For example, the b* and p* kana look exactly like the corresponding h* kana, aside from an additional diacritic mark: ハ, バ, and パ. I attempted to capture symmetries in each vocabulary set to make those kana easier to learn. However, there are times when purported symmetries fail. Take, for example, the t* kana and their transliterations: タ (ta), テ (te), ト (to), チ (chi! not “ti”), ツ (tsu! not “tu”) Not all of the transliterations in this set begin with “t”. In fact, there are no kana commonly transliterated as “ti” or “tu”. A different asymmetry exists in other kana. Instead of an exhibiting unexpected transliterations, some of the kana seemingly fail to exist: "yi", "ye", and "wu" fail to exist (see https://en.wikipedia.org/wiki/Katakana#Yi,_Ye_and_Wu). The other w* kana, while sometimes used, are largely obsolete. "wo" is retained as a particle and typically written in its hiragana form, while "wi" and "we" have been supplanted by their respective vowels, "i" and "e".
Vocabulary Set: Trick Questions and False Symmetries
"yi", "ye", and "wu"
These kana do not appear in modern Japanese orthography.
"ti" and "tu"
These kana transliterations are not commonly used.
"chi" and "tsu"
These kana transliterations are commonly used for チ and ツ, respectively.
These kana are largely obsolete, typically written in hirigana form, retained for use as particles, and pronounced as their vowel counterparts.
ヰ ("wi") and ヱ ("we")
These kana are largely obslete and pronounced as their vowel counterparts.