A letter is a character from an alphabet. Letters can be read. A letter usually corresponds to a sound that can be heard. For example, the letter A stands for the sound A, as in the beginning of the word "monkey".
Spelling means to break down the written word into its smallest units. This is how you can type a word into a computer. A lot of letters together make a book, hence the name.
But there are more different sounds than letters in our alphabet. That's why one letter is often pronounced differently. This has to do with where in the word the letter appears. The "O" in the word "oven" does not sound the same as in the word "open." The German S is usually voiced at the beginning of a syllable, so it is soft, as in "sanft." Otherwise, it is sharp, as in "ice." Other sounds are formed from several letters, such as the "sch" in "Schule" or the "ch" in "Chinese."
The German script has a lowercase and an uppercase character for letters. The upper case letters are used at the beginning of a sentence and at the beginning of nouns or in names like "Eidgenössische Technische Hochschule". The lowercase letters are used for the rest of the text.