Hiragana and Katana, together referred to as called Kana, are often called syllabaries, meaning that they’re sets of symbols that describe syllables. That’s not quite right, because they aren’t describing syllables, but morae. A syllable has to do with one’s breath. A mora (singular of morae) has to do with the sound in the syllable. Take any sentence, and say it in a whisper and say it mute, syllable by syllable. The breaths that you’re expelling are the syllables. The sounds, which you aren’t making, are the morae.
In Japanese, we can have a sentence like this: “Kyūshū wa toshi ga chiisai.” If we do the same exercise and thus divide the sentence by syllables, we’ll get “Kyuu-shuu-wa-to-shi-ga-chii-sai.” (8 syllables) But what the Kana does is divide it by morae: “きゅうーしゅうーはーとーしーがーちいーさい.” (12 Kana) So there are more Kana than syllables.
Hiragana and Katakana have near perfect equivalency, meaning that almost everything you can write with one, you can write with the other (the exceptions are very minor technical things.) Every あ (Hiragana) can be switched out with ア (Katakana), every い with イ, etc. So if you know how to work with one of them, you know how to work with the other.
Hiragana is the “smooth kana.” It’s the more common of the two (not by a lot, but it is.) Hiragana’s main function in day-to-day business is to serve as “okurigana,” which are the prefixes and suffixes that are attached to lexical stems. Imagine that the word “destined” was Japanese for a second. The /destin/ would be the lexical stem. It’s where the meat and potatoes of the meaning lies; and we see it in a lot of other words, like “destiny” and “destination.” The the /ed/ is just a suffix. This qualifies the word and makes it a specific part of speech. The /destin/ would be written in Kanji and the /ed/ would be written in Hiragana.
When one lengthens vowels in Hiragana, one tends to write the character including the vowel alongside the sole vowel/morae. For example: “ちいさい” begins with “chii,” two /i/ vowels. So you write ち, which we romanize as “chi,” and add い, which is romanized as “i.” The lengthening of the /o/ vowel tends to be written with an /u/; but there are words, especially older words, where the lengthening happens with two /o/’s, which as とお, “too,” meaning “ten.”
Children, or people who are in the process of learning Kanji, write almost everything in Hiragana. A special function of Hiragana is to make things look a bit more effeminate. So one may opt to write something entirely in Hiragana just for that aesthetic.
Katakana is the “fragmentary kana,” which refers to how a lot of it is sharp lines and how in writing it’s nice to have some kind of flow (like the curves in Hiragana).
Katakana has a number of functions. It’s most famously used for loanwords and foreign names, which are quite a lot in Japanese. It’s also used in textbooks for readings of Kanji; and, as an extension to that, one finds it as an explanation of the reading of a proper name in Kanji.
Vowel lengthening in Katakana tends to occur with a dash (ー), called a “chounpu” in Japanese. So “chii” in Katakana is not チイ but チー. The dash is sometimes seen in Hiragana for stylistic reasons, particularly if one wants to note the pronunciation of something. For example, there’s an expression in Japanese, “しつれいします,” “shitsurei shimasu,” used when one is entering a superior’s office. Japanese people sustain that last /a/, so it sounds like “shitsurei shimaaaasu.” So someone may decide to write that as しつれいしまーす.”
It’s also used when you want to make things sound more masculine. So in the same way that you might right something entirely in Hiragana to make it sound more feminine, you might write something in Katakana to make it sound more masculine.
Certain kana take a symbol called “dakuten,” or voicing maker (the two lines in が.) These are the kana whose syllable is /k/, /t/, /s/, and /h./ Voicing is a phenomenon in phonetics that makes it so that a consonant whose elocution does not involve the moving of the vocal cords to now use the vocal cords.
/k/ becomes /g/, /t/ becomes /d/, /s/ becomes /z/, and /h/ (the exception to this) becomes /b/. This is because once upon a time /h/ was /p/; and the voiced counterpart to /p/ is /b/.
The /p/ consonant still exists in Japanese, though it is probably the rarest of consonants. To write a mora with the /p/ consonant, one takes the /h/ morae with the same vowel and adds a “handakuten,” the small circle in ぱ.
Morae can also be palatalized, which means that they’re pronounced at the palate of the mouth, another phonetic phenomenon; and that involves the writing of a small や、よ、or ゆ next to the mora that can be palatalized. We romanize this as the main consonant plus a /y/ and the vowel. So は (“ha”) is palatalized to ひゃ (“hya”).
The /w/ is slowly disappearing in Japanese. The mora を, “wo,” is now pronounced as a pure “o.” There are Hiragana and Katakana for “we” and “wi,” (Hiragana: ゑ and ゐ; Katakana: ヱ and ヰ) but they came into disuse only 2 generations ago.
Also, from this you’ve probably gathered that Japanese’s set of sounds is quite small. That is true. There are about 100 different morae in Japanese; and like 50 of them are variations of the basic ones (through voicing and palatalization). There are languages with fewer phonetic sets, like Hawaiian, but this is definitely small, much smaller than English.
Also worth noting: Japanese people, when whispering, divide the sentence into morae. It’s just a habit of theirs; but when they speak normally, the syllables match what we talked about.
So that’s all I have to say right now about Kana. Let’s talk about Kanji!