When computing started out, memory was limited, so not many letters were provided. Different ways of representing characters arose, making it difficult to exchange documents.
The Unicode Consortium provides a standard way of encoding characters, and has the ambitious task of assigning a unique number to every character in human languages. If you go to the Code Charts page and open a chart, you’ll see each letter has a four-digit code below it. That’s the Unicode hexadecimal code for that character. (Hexadecimal is a special way of counting.)
One way of entering Unicode is to hold down the Alt key while typing the hexadecimal number. For example, hold down Alt while typing 0101, then let go. An “e” will appear. Hold down Alt while typing 0154, then let go. An “š” will appear.
Unfortunately, this method does not always work. FileFormat has provided a free utility for typing in Unicode characters using this method. Warning: I haven’t personally tested this, yet.