Plagiarism means the complete copying of someone else's work and presenting it as one's own. Plagiarism includes but not limited to using a quote by someone without a proper reference to the owner of such quote, producing a literary work of another person, copying of a piece of writing by another person without a proper reference to the originator, just to mention but a few.
Look at the following sentences "The debate about which of Cristiano Ronaldo or Lionel Messi is the better player isn't exactly a new one and the Portuguese himself has been known to have an opinion on the comparison.
But, according to Peter Crouch, Ronaldo's opinion was also determined by factors that had little to do with either player's ability on the pitch."
The sentences above were gotten from source; so now if I come up with a write up and I include these sentences without a proper reference to the source and without any changes to the original context as written by the originator, I'll be accussed of plagiarism. The act of doing this and presenting it as one's own work is referred to as plagiarism. Anyone who is accused of plagiarism is known as as plagiarist and the work itself is known as a plagiarized content.
Plagiarism is a big offence when reference is not made to the original writer or the owner of the content. But be that as it may, There is no way one can be a perfect writer and one can never be a Mr. Know-all, but anytime a writer wants to use a work of others that has been previously used, it's better to reference it so as to give credit to the original owner of such writings.
Check out the image below, I can't make use of the photo in any write up unless I make a proper citation as per where the image was gotten from since I'm not a photographer and I've never watched a live football match talk more of seeing Cristiano Ronaldo play in a live match.

source

In summary, plagiarism can be seen as the stealing of another person's work and efforts and that's really bad; as it is recommended by steemit.com, every user must avoid the use of plagiarized content in their works as this will give more credibility to this platform and anyone who visits steemit will be sure of getting original contents.

"I have read and understood the Steemit Etiquette on Steemit Community and will do my best to embrace them"

Thanks.