For further information on copyrighted materials please read section Theses and Copyright, Examples of copyrighted materials.
Websites
Websites usually have a set of Terms & Conditions (T&Cs) of usage, telling you what you can and can't do. Whilst some websites might state you cannot copy, it is legally permissible to copy an insubstantial amount under Fair Dealing (read Fair Dealing and Copying Legally section for more information)
Other considerations are
Electronic Resources
Online journals, databases and Ebooks usually have a licence determining who the users are and what they can do with the resource. Points 1-3 will usually apply but please read section Electronic Resources for more information.