An essential legal reason to acknowledge your sources is to avoid plagiarism. Plagiarism is a serious academic offence. It means that you present others’ work as your own without clearly acknowledging the source of information. Any ideas or data which are not your own must be cited, including statistics, tables, figures, pictures, illustrations etc. For electronic sources on the World Wide Web (Internet) the same rules apply as to printed sources.
So be careful, if you fail to properly cite a source (even if unintentionally), it is considered plagiarism. Information of a general nature which is considered common knowledge, need not to be cited, e.g. it is common knowledge that nobody is in charge of the World Wide Web and anybody can publish on the World Wide Web. If you are in doubt whether it is considered common knowledge, it is better to cite a source than not.
Read the Wageningen University regulations on plagiarism in the Student Charter 2011-2012: Education and Examination Regulations for the Bachelor’s study programmes or Master’s study programmes (Art. 32 Use of plagiarism scanners and Art. 33 Fraud and misconduct: prohibitions). These documents are also available in Dutch (Onderwijs en examenregeling bachelor en masteropleidingen).