No one controls what can be posted on the web, but research articles and other published materials have often gone through a rigorous review process.
What can be confusing is how publishing has been changing. Some research materials may only be available
online. Many libraries no longer subscribe to the paper version of some periodicals,
but subscribe instead to the online version of the journal or rely on
full text access through Web-based periodical databases.
When teachers tell you they don't want you to use Web pages to do research, they don't mean (or shouldn't mean) you can't use the Web to do your research. Compare the results between a regular Google search and a search using Google Scholar.
The online version of a periodical article is no more or less reliable
than the same article found in the print version of the journal. If you find a periodical article on the Web, however, you might not be able to easily see if it was orginally published in a magazine with a glossy cover that
was filled with advertising or in a journal that is intended for an academic or professional audience. It can often take a bit of an effort to distinguish among magazines,
trade journals, and peer reviewed journals.