The CHAR() function can be used to represent a character in a cell if you know its ASCII number. For most characters, it’s a heck of a lot easier to use those letter keys on your keyboard, but it can be useful for some other characters.
For instance, if you want to put an address in cell on multiple lines, you can use CHAR(10) (10 being the ASCII code for a line feed).
If you get a bunch of squares and your text is all on one line, go to Format > Cells > Alignment and check the Wrap Text box.
To create a line feed while typing (i.e., not in a formula), Alt+Enter does the same thing.