A phonogram is a grapheme (written character) which represents a phoneme (speech sound) or combination of phonemes, such as the letters of the Latin alphabet or the Japanese kana. For example, "igh" is an English-language phonogram that represents the hard "I" sound in "high." Whereas the word phonics refers to the sounds, the word phonogram refers to the letter(s) that represent that sound.
This contrasts with logograms, which represents words and morphemes (meaningful units of language), and determinatives, silent characters used to mark semantic categories. They are letters.