In last week's lesson, you learned about expanding your vocabulary. Good writing depends on more than just having a large vocabulary, however. You have to learn to use the vocabulary correctly in the right contexts. Choosing the right vocabulary is also known as choosing your diction.

 If you know the word dictionary, then you have seen the word diction. In fact, diction is simply the words you choose to express your meaning. However, choosing the right words isn't always so simple.

The words you choose can be formal, informal, or colloquial/slang. Using slang when you should be writing formally is a big mistake. Similarly, if you are overly formal in a situation that calls for informality, you risk insulting your reader as well. 

book Formal Informal Slang
I am angry.  I'm mad.  I'm pissed off. 
(slightly vulgar)
my father my dad   my old man
What you are proposing does not seem feasible.  I don't think your plan sounds all that possible. That plan's crap. (slightly vulgar)
It was a pleasure meeting you.  See you around.  Later, dude.