Steemit is a social media platform unlike Facebook & Twitter. Here you get paid for the content you post.
Copy & Paste
Copying & Pasting content even with sources or attribution does not give your post value. It is considered spam since the same content is often shared by multiple accounts. Adding your own thoughts and opinions to the content gives your post value. It will also encourage users to comment and engage them long term.
Plagiarism
In other words, plagiarism is an act of fraud. It involves both stealing someone else’s work and lying about it afterward.
Most cases of plagiarism can be avoided, however, by citing sources. Simply acknowledging that certain material has been borrowed and providing your audience with the information necessary to find that source is usually enough to prevent plagiarism.
What is Plagiarism?
Stealing Content
Copying someone else’s work 100% while claiming it as your own or letting others believe it is your own work in order to earn rewards is unacceptable in most societies. Here it will get you flagged and added to 
Improper Citation
This is social media not a professional journal or newspaper so the the requirements for citing and determining what is “common knowledge” are pretty lax within the community with the exception of some curation groups. 
Translations, Spun Content, Re-write
All three are grouped together as it involves taking a piece of content and transforming it.
Translations
With few exceptions translations of texts created by others is considered spam. Anyone can take a text from the internet and use a translation software.
Exceptions:
Translating your own content
Translating others content at their request or with their permission
Spun Content
Content spinners are popular SEO tools to create junk websites and drive traffic to them. These programs range from simply replacing words or more advanced tools rearrange sentences. This is plagiarism and considered more offensive than just copying text as it shows the intent of trying to hide it.
Rewrite
This is a little more complex. If you are simply replacing words and switching a few things around then it is the same as spun content.
If you do a complete rewrite- different sentence structure, added/removed information, added your own thoughts and opinions then it could be considered original. If the content doesn’t include what is considered “common knowledge” a source should still be included.
CC0, Public Domain, PLR or Requires No Attribution?
Did you create it?
Ghost Writers
This is a bit of a grey area. Many people hire professional content writers. While it would be good to acknowledge the content isn’t yours by adding a ghost writer by line it isn’t required. What is required is to make sure the content is original and not stolen, spun, or poorly rewritten. You are responsible for the content you post.
General tips to share content and add value:
Using a few sentences from your source in “quotes.” Use HTML tags or Markdown.
Linking to your source, even if it isn’t the original source.
Include your own original thoughts and ideas on what you have shared.
Not a native English speaker? Write in your own language.
Write twice as much as you quote.
Ask questions to engage your readers.