Attempting to pass off someone else's work as your own is plagiarism.
You may be encouraged to use the Web as a resource for writing assignments. This does not mean you should copy chunks of text from other websites however. You can quote from other sites, but such quotes should always be acknowledged. You should write material in your own words, to demonstrate that you have understood it, rather than simply copying it. Using search engines it is relatively easy for markers to find text taken from another site.
You should always acknowledge the source of any 'third party' material you include in your own work. Failure to do so will be interpreted as a deliberate attempt at plagiarism. Plagiarism is, as a rule, taken very seriously. At the OU, for example, when detected, plagiarism can result in a range of measures from receiving a zero score for the whole assignment to being expelled from the course. It is your responsibility to ensure that your assignments are properly referenced and are your own work.
A guide on how to correctly reference sources of information, including online sources, can be found on the OU Library's website.