In languages that use an alphabet, for example English, each symbol in the alphabet is a letter. Letters represent sounds when the language is spoken.
Some languages do not use letters for writing: Chinese, for example, uses "ideograms".
In English and many other languages, the smallest part of writing is the letter. We use letters to make words. In some languages, for example Spanish, there is usually one letter for one sound (or better: for one phoneme, the smallest part of speech). This is easy to read. In other languages, for example English, we can use several different letters for the same sound in different words, or one letter for different sounds in different words. This is difficult for learners to read. Here are some examples: