Here's a simple PPT I put together to help you understand the basics of twitter - there's nothing about trending - but you'll see there is an explanation of some simple concepts. Hope it helps :-)

1. TWITTER
WHAT TWITTER DIDN’T
TEACH YOU
No copyright to TIA KANSARA

2. WHAT DO YOU THINK
TWITTER IS?

3. TWITTER IS A REAL-TIME SMS
SERVICE FOR FREE - THAT
EVERYONE SEES
EVERY TWEET = SMS

4. Text messages that
everyone can see on
your page, but also on
theirs (feeds)
Having a topic you are
discussing that others
can find (#)
Following people you like
(Click follow) and directing a
message to them (@)