I remember when I had to learn all these; it was definitely a challenge. When I first started seeing these, all I saw were dots and lines, with no 'emotion' behind them, and since e both have Asperger's, it wouldn't surprise me if that is what you are seeing too.

Here's an explanation of them and not specially what they are…

:P - Is a smiley face with the tongue out. Often you will see these when someone is attempting to express that they are being silly, teasing, or joking around, otherwise not to be taken seriously. "Haha! You lose! :P" (Also can be seen as >:P which is a more malicious but still a silly smile: "I threw a snowball at Lindsay >:P.) The tongue smiley is harmless teasing or supposed to be for humor.

:/ - Represents displeasure but not anger. More like annoyance, displeasure, or concern. "I have a test on Monday; I hope I do well :/," or, "I hate it when there's so many mosquitos :/."

XD - This one eludes me too but I do know what it means. I don't know how the 'X' represents a person's face, however the emotion is to represent a big smiley face, like when someone is laughing or smiling heavily. "My best friend's spelling always makes me laugh xD."

Hope this helped . (Now I used emoticons almost obsessively out of habit.)