1. 音位 (phoneme)

https://en.wikipedia.org/wiki/Phoneme

In linguistics and specifically phonology, a phoneme (/ˈfoʊniːm/) is any set of similar phones (speech sounds) that, within a given language, is perceptually regarded as a single distinct sound and helps distinguish one word from another.
音位是一种语言中能够区别词之间意义的最小语音单位，其最重要的特性是它与该语音系统中的其他音位相对立，也是该特定语言的语音系统中最小对立单位。

音位可概分为元音及辅音两类，或由声带的振动与否分成有声音及无声音两类，是音位学分析的基础概念。

在语言学当中，斜线 (/ /) 表示使用音位标音法，音位就在两条斜线之间；而方括号 ([ ]) 表示语音标音。音位标音 (phonemic transcription) 只记录音位，音位标音法只是宽式标音的一种。

1.1. Correspondence between letters and phonemes

Phonemes are considered to be the basis for alphabetic writing systems.

The correspondence between symbols and phonemes in alphabetic writing systems is not necessarily a one-to-one correspondence. A phoneme might be represented by a combination of two or more letters, like ⟨sh⟩ in English or ⟨sch⟩ in German (both representing the phoneme /ʃ/). Also a single letter may represent two phonemes, as in English ⟨x⟩ representing /gz/ or /ks/.

2. 元音 (vowel)

https://en.wikipedia.org/wiki/Vowel

元音 (又称母音) 是音位的一种，与辅音 (又称子音) 相对。

A vowel is a syllabic speech sound pronounced without any stricture in the vocal tract. Vowels are one of the two principal classes of speech sounds, the other being the consonant.
元音是在发音过程中由气流通过口腔而不受阻碍发出的音。发元音时，气流从肺部通过声门冲击声带，使声带发出均匀震动，然后震音气流不受阻碍地通过口腔、鼻腔，通过舌、唇的调节而发出不同的声音。

3. 辅音 (consonant)

https://en.wikipedia.org/wiki/Consonant

辅音 (又称子音)，在调音语音学中是音位的一种，与元音相对，发音的调音环节中气流在调音器官某一部分受到完全或部分阻碍。

3.1. Consonant sounds and consonant letters

The word consonant may be used ambiguously for both speech sounds and the letters of the alphabet used to write them. In English, these letters are B, C, D, F, G, J, K, L, M, N, P, Q, S, T, V, X, Z and often H, R, W, Y.

In English orthography, the letters H, R, W, Y and the digraph GH are used for both consonants and vowels. For instance, the letter Y stands for the consonant/semi-vowel /j/ in yoke, the vowel /ɪ/ in myth, the vowel /i/ in funny, the diphthong /aɪ/ in sky, and forms several digraphs for other diphthongs, such as say, boy, key.

4. 元音字母 (vowel letter)

元音字母 (又称母音字母) 是指语言里起着发声作用的字母。元音是音标，写在括号里面。元音字母是字母，它发的音是元音。

英语字母一共有 26 个，其中有 5 个元音字母，19 个辅音字母和 2 个半元音字母。元音字母有 a，e，i，o，u。元音字母是指发音为元音的英语字母。元音字母在不同的词里面可以发不同的音，即会发出不同的元音音标。

英语音标一共有 48 个音素，其中有 20 个元音音素，28 个辅音音素。元音是指元音音标，即在发声的时候感受不到气流的阻碍的英语音标。

References

[1] Yongqiang Cheng, https://yongqiang.blog.csdn.net/