Plagiarism is the use, as one's own, of work of another person without proper acknowledgment. So if you copy passages from a book or webpage in your essay without saying so, it counts as plagiarism.
Plagiarism is a very serious offence and can get you expelled from the university. So make sure you understand what counts as plagiarism. If you submit your written work through turnitin, make sure you check your originality score. A similarity index of 25% or above is unlikely to be acceptable.
HKU has a booklet about plagiarism: http://www.rss.hku.hk/plagiarism/
Read the following carefully:
Suppose a student writes such a passage:
@Rey attacks the theory by arguing if all it takes to make a first-order state a conscious state is that the state be monitored by a scanner that makes integrative use of the information thus gleaned, then the Inner Sense theory would be far too weak@
This is very bad. The bold text is copied from a paper without acknowledgement. This is plagiarism and you can get zero marks as a result. If you submit your work through turnitin, make sure you check your originality score. A similarity index of 25% or above is unlikely to be acceptable.
When you copy a sentence or a long phrase from a text and use it in your essay, you should use quotation marks. Here is an example:
@Is a spider conscious? We cannot imagine a consciousness very different at all from our own, much less a greatly impoverished one. 1 Therefore, we can never know whether other creatures other conscious. @
Footnote 1. Block et. el. ...
The second sentence in boldface is actually not written by the student, but copied from a paper. Although the source is given in a footnote, this is not enough. If you have to include the sentence, write like this:
@Is a spider conscious? Not according to Block et. el. (1994). They say, "We cannot imagine a consciousness very different at all from our own, much less a greatly impoverished one" 1. If they are right, we can never know whether other creatures other conscious.@
Footnote 1. Block et. el. ...
But try to avoid writing like that. You should use your own words where possible.
The following is from a book:
@For that matter, individual subsystems of our own human psychologies doubtless involve their own internal monitors, and it is implausible to grant that those subsystems are themselves conscious.@
This is from a student's essay:
@we can consider the individual subsystems of our own human psychologies. They doubtlessly involve their own internal monitors, but obviously it is implausible to claim that those subsystems are themselves conscious.@
This is very bad, a case of cut-and-paste. Apart from changing a few words, basically they are the same passage. This is not how you should write an essay. Even if you put the book in your list of references, this is still plagiarism. You need to use your own words to explain those ideas. Or use indentation to quote the passage exactly and give a proper citation.