In modern Japanese, four different types of characters are used.
The Kanji characters allegedly came to Japan in the late 4th century via a Korean scholar with Chinese Confucian texts. They usually have at least two readings (pronunciations). On the one hand, the original Chinese term in Japanese pronunciation, and then the Japanese equivalent of that term. As an example let's look at the character 道 from Aikido. From the Chinese comes the reading Dō, which means way or method in Confucian texts. The Japanese word for way/street is michi. For michi was then used the chinese character. From pronunciation alone you often can not clearly identify the term. First example: There are more than 4 dozen characters with different meanings which are all pronounced ki. Aikidoka know the 気 or in traditional writing 氣. A tree is ki 木 in Japanese, 汽 is water vapor, 機 plane or machine, etc. Second example: In Aikido we know the word uchi, as in yokomen uchi. There the meaning is blow/strike (打ち). With kaiten nage there is the form uchi (内), i.e. inside, when you step forward under uke's arm. But the Japanese word for "at home" is also uchi and is written with the Kanji 家.
Since the last orthographic reform in 2010, there are 2,136 official Kanji, set by the Japanese Ministry of Education. There are more than 50,000 kanji in total.
When writing kanji by hand you have to follow a certain order and direction for the strokes. There are some basic rules for the correct sequence, but often you have to just learn it by heart. The order of the strokes becomes especially important when the kanji are written fluently.
The syllabary Hiragana was created in the 9th century. The characters are derived from kanji, according to the phonetic notation. Especially in the courtly literature, this "alphabet" was quickly spread. Japanese children read and write hiragana at school first. A text completely written in Hiragana without Kanji, however, gets a bit difficult to read and sometimes there are ambiguities.
In normal texts, hiragana - roughly speaking - is used for grammar and for characters that are too complicated. Even in private letters, a lot of hiragana was used, as it was considered impolite to want to impress the recipient through ones own education.
The picture shows the famous poem iroha, which contains each hiragana character of the Heian period exactly once (we and wi are no longer used today and the newer n is not included). The poem has traditionally been used for lexical sorting.
Katakana is the second Japanese mora-script ("syllabary-script"). It was developed in the Heian period (794-1185) to make Chinese texts more readable. In the Meiji period (1868-1912) the script was still used "for grammar", especially in documents and official documents. This can still be seen in our DEN and DAN certificates.
The most important application for Katakana today are foreign language terms. Most new foreign words in Japanese come from English. Also foreign-language proper names are written with Katakana, we often see this embroidered on Hakama or on the jacket of the Dōgi.
The re-translation back into the source language sometimes turns out to be adventurous. But for Japanese people, this transcription has a decisive advantage: The Katakana provide an easily comprehensible pronunciation. They don't have to think twice about whether the word comes from English, Spanish, French or some other language and then come up with how to pronounce it.
The picture shows the nameplate of a restaurant in Kobe, taken by ralf_treiner. For Germans it may be strange to read "der Hase" (the rabbit) in a sort of phonetics, because one is to much focused on the letters.
But take the example from above スペシャルインタビュー (supesharuintabyū): The u-vowel in Japanese is weak, the ru is near to a lu, thus supesharu becomes in reality speshal with is quite near to special. In former decades the official Japanese tried to use only genuine Japanese sounds in Katakana, therefore the byū was the best approximation for view, and we get easily intaviu out of intabyū, which is interview in the original language.
Until establishing Japanese writing on electronic devices, it was easier to write a text by hand rather than using any technical aids. For letterpress and newspapers there were complex typesetting machines. For computers, "word processors" were developed. Today it is very easy to write a Japanese text on the smartphone. All you have to do is select the Japanese keyboard and understand the logic of the input.
The figure shows the process on the PC. You will use your standard keyboard (German, English, Italian etc.) and select first the language Japanese for writing on the PC. You can then choose between Katakana and Hiragana or even switch between them. The Japanese word is beeing typed in romaji i.e. in Latin letters on the keyboard, e.g. aikido.
As soon as you have typed ai, the syllables appear on the screen (here in Hiragana) with a list of possible words that you seem to want to write. If you add ki to it, the selection changes. The third figure shows the result after entering aikido, still without the u of the long o. Now you can select the word and you can continue writing.
The result of the electronic aids is that the younger generation can no longer write all necessary kanji by hand, they know them only passively.