Examples of plagiarism are the following:
1-Total plagiarism: cut-copy sentences and paragraphs from other publication without the corresponding citation
2-Mosaic plagiarism: this type of plagiarism is the result of copying some passages (sentence fragments) of distinct publications without citation.
3-Self-plagiarism: it is also a copy-paste case derived from the reuse of already published papers from your own.
All the cases above described are serious cases of plagiarism, while incorrect citations to previously published works enter into consideration as 'minor' cases of plagiarism or considered as an error during the publication process.