I feel like I should make something clear after thinking about this:
"" indicates a letter and nothing more (as we know that in English letters aren't tied to one sound, or even two in some cases)
// indicates a phoneme -- this is pretty much the idea of a sound as it exists in a language
[] indicates a phone -- the actual sound that is produced
To use the example from above to help illustrate this, the phoneme /t/ in English is represented by the letter "t", and can be realized as the phones [tʰ] ("top", "till"), [t] ("stop", "still"), [t(
unreleased)] ("rat", "mutt"), and [ɾ] ("butter", "sitting"). The last one may not happen in some dialects of English, though, and can instead be [ʔ] (voiceless glottal stop).
I just want to make sure the terms are clearly defined to reduce confusion.