A common source of unintentional plagiarism is poor paraphrasing. Good paraphrasing requires a fundamental understanding of the ideas of the source material. Shallow or incomplete reading of the original text might not result in plagiarism, strictly speaking, but it could result in a misrepresentation of that text - another kind of academic irresponsibility.
The ideas of the source material must be stated in your own words. You are plagiarizing if:
Purdue OWL has a paraphrasing guide.
The following video offers some practical advice in paraphrasing.