When producing a piece of written work for college or a report for your boss it is good practice not to repeat the same word too many times in your text. Show that you have the knowledge to say things in other ways. This makes what you have written more interesting to read as well as enabling you to achieve a higher score in your studies.
Looking at this text as an example, how many different ways are there to say 'dog'?
'A few months ago, Buster the Border collie was rescued from the local animal shelter and given to the Baily family.'
'"Oh, Bob! You shouldn't have!" Charlotte said as soon as he surprised them with the canine. "What's up with the pooch?" Connor said when he came to see what the ruckus was about. "He's all yours." Bob said. "Your mum is always talking about the Border collie she had when she was a little girl and said she's always secretly wanted another one."'
-----Extract from 'The Land of Stories the Enchantress Returns' by Chris Colfer-----
In fact, the word 'dog' is not used at all!
Other words that could have been used are:
puppy (depending on the age)
Just like with this extract, the best technique to increase your vocabulary is by reading.
The more you spend time absorbing different texts the better you will become at finding new ways to say the same thing.