Text in number cells can be entered as a string. Strings must be enclosed between double quotes. They can contain spaces, but not control characters.
"As long as a piece of string"
If you need to include a double quote mark within a string, you must enter it twice:
"She said ""Hello!"""
It is much easier to enter text strings as plain text in a text cell. When you need to use the text in a formula, you can enter its cell reference; this automatically creates a string out of the contents of the cell.
To enter text in a cell, place the caret within the cell and type the text. If the text was not entered via the formula line, it will be interpreted as text automatically.