In Japan, we are taught how to read and write Katakana ( カタカナ ) during the first grade (Age 6). Once you got it, you would be able to write a letter or mail in Japanese however the face of the mail, that written in Japanese used only Katakana, would be unnatural. The reason why it seems to be bizarre is that Katakana ( カタカナ ) is supposed to be used to basically describe Imported words.

Here is an example.
Let’s say, if you want to say that you ate an omelet this morning.

If you only use Hiragana to write this, it would be
(A) わたし　は　けさ　おむれつ　を　たべた 
[watasi wa kesa omuretu wo tabeta].

Then, if you replace some words with Kanji character, it would be
(B) 私　は　今朝　おむれつ　を　食べた 
[watasi wa kesa omuretu wo tabeta].

And then, if you replace an imported word with Katakana, it would be
(C) 私　は　今朝　オムレツ　を　食べた 
[watasi wa kesa omuretu wo tabeta].

Even though the face of (C) is different from the face of (A) and (B), all of them are exactly the same meaning.

What is difference between (A) and (B) is that 4 words are replace to Kanji characters. And, what is difference between (B) and (C) is that 1 word is replace to Katakana characters. If you see a sentence (C), you will notice that there are some Hiragana letters and wonder why these letters are still written in Hiragana. The answer to your question is that these are the letters that are not replaceable to Kanji and Katakana letters.

I might have written a lot about Japanese letters though; I hope that you are still with me.

What you would see in this app is a list of Katakana letters. If you tap one of those letters, you can practice how to draw its letter. After practicing to draw a single Katakana letter, you would also practice many imported words written in Katakana as well.

Enjoy drawing Katakana letters!