The rules for being polite while you're online are known as netiquette. It's bad netiquette to type in all capital letters, because it seems like you're shouting!
The portmanteau netiquette was coined in the late 1980s or early 1990s from internet and etiquette. While people differ on the exact details, most internet users agree that there are certain standards for acceptable online behavior and style. Hostile language, spreading rumors or private information, and dominating forum discussions are all considered poor netiquette. On the other hand, treating people with respect, avoiding spam, and communicating clearly are examples of excellent netiquette.