Twitter has a primer somewhere that explains all the abbreviations. Twitter is basically a "micro-blogging" site, so everyone's page is like a blog consisting of 140 character posts. That page that is linked to here doesn't actually show Malibu's message, it shows the dude's response to her comment. Her message would show up on her own Twitter page, with an @ mention of the guy she's talking to. So, one person's Twitter feed only shows their own comments, I believe. When you talk about someone else on Twitter, you put the @Username, and then Twitter alerts that person they have been talked about. So when the @malibunextyear appears on the guy's page, that is his response to her @mention of him. To see what she wrote, we'd have to go to her twitter page...I think...maybe I don't explain it or understand it very well though.
The # signs are topic identifiers, so #SGU would mean you are talking about SGU, and you could tag your Tweet with #SGU if you wanted it to count towards the "trending" topics (the tags that show up the most, are 'hot' topics). And the RT means you are retweeting (basically copying someone's tweet and posting it on your own Twitter page - but you are supposed to RT cuz it helps spread the word about things).
Anyways I don't have my own twitter account but I had to read up on how Twitter works recently for a work thing. Maybe someone who actually uses it all the time can explain better