What is phonetics?
It is the branch of linguistics that study the different sounds in a language by using written symbols that help in the production, description, combination ,and representation of the words.
What is a phoneme?
It is the smallest unit of sound within a spoke word. (Roach, 1998)
Even though there are 25 letters in the English language, it has 44 phonemes because from one letter there are many sounds available.
References
Roach, P. (1998). English phonetics and phonology: A practical course, second edition. Cambridge university press.