Since its invention, language has gradually moved from being primarily a spoken medium to a text-based one, as in the words you're reading now. Communication by text has grown by leaps and bounds in recent years, as people forego phone calls to text or instant-message. But, as Ben Crair notes in a recent article in the New Republic, a new phenomenon in how we communicate has been born: Punctuation that was once completely neutral is now seen as having inherent emotion. Take the period (like the one at the end of this sentence). It was one of the first punctuation marks, and its purpose was to note a definitive pause, as it was first used to mimic speech.
Now, though, a period can make the difference between a message seeming calm or seeming angry. Crair uses the example of a boyfriend, tired from work, asking whether, instead of eating at a restaurant for his girlfriend's birthday that night, they can eat at home. Her answer of "we could do that" is laid back. The answer becomes passive-aggressive, however, if it is "we could do that.", he writes. There's even data to back it up. A recent American University survey of college students found that only 39 percent used final sentence punctuation in texts, and 45 percent for online chats. There are other signs of unprecedented emotional impact in punctuation. The exclamation point is now used as a sign of sincerity and enthusiasm, as in "I love it!" As textual language continues to grow, expect more changes in the future.
For some further instruction on punctuation, check out this video of LL Cool J from "The Electric Company."