Inserting an external link into an article to show where you got information is better than nothing, but by itself it’s not the proper way to cite a source. An embedded link (an external link in the middle of an article) isn’t a proper citation because links, like milk, have a tendency to go bad over time. Links can stop working when a Web site goes out of business, someone moves or deletes a Web page you linked to, or a URL changes for any number of reasons. When links go bad, so does any substantiation of the sentences that the links were supposed to support.
The general problem of links going bad is called link rot. A non-working link without any other information is almost worthless. Page 349 discusses some ways to try to fix a bad link.
The best way to reduce the impact of bad links is to fully cite your source, to include more information than just the URL.
Currently, Wikipedia lets you use one of the three different methods for a proper citation. All three use a “References” section at the bottom of the article.
Embedded citations. You put an embedded link into the article (as described earlier), and then put the same URL, plus additional information about the source, into the “References” section. For details on this method, go to WP:ECITE.
Footnotes. You add all the source information into the body of the article, plus special footnote markup. When the Wikipedia software displays the article, it puts a footnote number in the body ...