I'm not sure what you mean.
It would be easy to tell you more than you want to know. The least you may want to know about phonemes is that they define the meaning bearing sorts of sound in a language: a bear is not a pear or a tear. But I'm talking here not just about 'letters'. The /l/ phoneme in 'leek' is not realized phonetically the same as the /l/ phoneme in 'keel'; the /k/ phoneme in 'leek' is not realized phonetically the same as the /k/ phoneme in 'keel'. The problem Japanese students have with /l/ and /r/ derives not from an inability to pronounce [l] and [r], but from a difficulty in recognizing them as different phonemes (which makes them difficult to pronounce in certain phonetic contexts, given a Japanese phonological background).
Look here (and hereabouts) Linguistics 001 -- Phonetics and Phonology for an overview of some of the issues.