Everyone will have their own reasons for using online genealogy websites. Some may want concrete data, others to see who is researching a particular family name. Whatever your reasons, you should always ask yourself how reliable the site is, before using the data it contains. Always check whether a source is given - such as an archive of origin or, better still, an archival reference - for any information provided. This will allow you to check the validity of the data.
'Unless your relative's name appears in the title of the document, it will not be found.'
Many archives, large and small, are placing their catalogues online and giving researchers the opportunity to search for information by typing in a keyword. Therefore you can, in theory, type in a relative's name and see if there are any documents relating to them.
This development is not as exciting as it first sounds, though, as usually it is only the document descriptions that are searched, not the document content. Unless your relative's name appears in the title of the document, it will not be found, even if that same name is constantly repeated in the text itself.
Remember to consider the question of copyright. Material that someone has placed online cannot simply be taken for your own use, especially if you are thinking of putting it on your own website. Attempt to contact the person who has taken the time and trouble to compile the site. In return you may find they can help you still further with your own researches.