On the Internet, you don't have important visual and auditory cues from a person like tone of voice or facial expressions. Body language, therefore, is non-existent - yet this can be very important in conversations. The people in the conversation may also not know each other very well; the relationships here are not akin to those in real life. They're casual and informal, and they may be completely global.
I've found that it's not always clear when someone is being sarcastic/ironic or not, and on Stack Exchange (in chat), this has caused some serious issues. It's sometimes hard to tell whether or not someone's joking. Misunderstanding this can have important consequences.
How can I indicate sarcasm or irony in a chat room without saying outright "I'm being sarcastic"?