What are these symbols?

I see this symbol (series of square blocks) quite often in Japanese Websites, on this site, e-mails, etc. Does anyone know what this is and why it appears? I've always wondered what it is and what causes it. It oftentimes block out meaningful messages.

�� �� �� �� �� �� �� �� �� �� �� �� �� �� ��
�� �� �� �� �� �� �� �� �� �� �� �� ��

Thank you in advance.

it means your browser encoding is wrong.

set the encoding to autodetect japanese encoding, Shift JIS usually is the best.

google how to change encoding on google to find out how to do it for your specific web browser

You may get a clear advice on how to cope with this phenomenon on your web browser if you show its name (and version).
If you are using Microsoft Internet Explorer 6 / 7 / 8, then open "View" menu, then open "Encoding" menu, then confirm which character set is selected, then move the pointer on "Japanese (Auto-Select)" and click it. If this phenomenon is not dissolved, then try another character set. Character sets used for Japanese characters include "Japanese (Shift-JIS)" / "Japanese (EUC)" / "Unicode (UTF-8)".

: Does anyone know what this is and why it appears? I've always wondered what it is and what causes it.

For characters to be handled by a computer, each character in a text has at least one character code and each character code has digital signs to mean it.
To display characters, a computer
(1) interprets digital signs as character codes, and
(2) replaces each character code by a character.
If (1) or (2) is not done correctly as intended, then characters may appear in wrong symbols, resulting in the phenomenon you often encounter.

This phenomenon, called "mojibake" in Japanese (and sometimes in English), may happen mainly as to non-English languages.
Some Japanese characters have different code positions in different character sets. If a writer inputs such characters in accordance with Character set A and a computer adopts Character set B for outputting, then they may result in mojibake.

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winterwolf and omotenashi:

Thank you very much for your clear explanations and how I can correct this problem. I'll do as you said. Thanks again!

David