There's nothing more convenient than Wikipedia if you're looking for some quick information, and when the stakes are low (you need a piece of information to settle a bet with your roommate, or you want to get a basic sense of what something means before starting more in-depth research), you may get what you need from Wikipedia. In fact, some instructors may advise their students to read entries for scientific concepts on Wikipedia as a way to begin understanding those concepts.
Nevertheless, when you're doing academic research, you should be extremely cautious about using Wikipedia. As its own disclaimer states, information on Wikipedia is contributed by anyone who wants to post material, and the expertise of the posters is not taken into consideration. Users may be reading information that is outdated or that has been posted by someone who is not an expert in the field or by someone who wishes to provide misinformation. (Case in point: Four years ago, an Expos student who was writing a paper about the limitations of Wikipedia posted a fictional entry for himself, stating that he was the mayor of a small town in China. Four years later, if you type in his name, or if you do a subject search on Wikipedia for mayors of towns in China, you will still find this fictional entry.) Some information on Wikipedia may well be accurate, but because experts do not review the site's entries, there is a considerable risk in relying on this source for your essays.
The fact that Wikipedia is not a reliable source for academic research doesn't mean that it's wrong to use basic reference materials when you're trying to familiarize yourself with a topic. In fact, the library is stocked with introductory materials, and the Harvard librarians can point you to specialized encyclopedias in different fields. These sources can be particularly useful when you need background information or context for a topic you're writing about.