The word 'plagiarism' is derived from the Latin 'plagiarius', a kidnapper.
This indicates how serious it is to use others' ideas and arguments without giving them full credit – it is regarded as a type of theft.
By plagiarising you are failing to give credit for other people's work and failing yourself – if you are able to paraphrase and put other's ideas into your words, you are showing that you have grasped their ideas and gained a higher level of understanding.
By correctly referencing you are showing in your assignment the sources of your ideas and arguments. You are also demonstrating that you have studied the appropriate work and can present your own ideas (and those of others) in a coherent manner, in your own words.
For guidance on the different forms of academic writing, please visit the Effective Learning Service web page.