Hands-on part: Use the internet to ...
A directory is a web page containing a catalog of thousands/millions of web pages, organized in some way. Whether or not a particular web page is included in some directory, and under what heading, is decided by humans.
With a search engine, you can search for web pages containing a given word or phrase wither in its description or even somewhere in the text (full text search). Of course this search is not done when you submit your query, but rather programs browse the net all the time and select the pages found in a huge database. Then your search just accesses this database.
Directories and search engines are almost always combined. Still some of the addresses given below are more directories (like Yahoo), others (like Google) are more search engines.
The WWW is a wonderful source of information, but not everything you find on it is true. Some of it is completely wrong. Not everybody can get his thoughts published by good publishers, but everybody can maintain a serious looking webpage.
Viruses are also usually caught over the internet.
Finally, if you use the internet for academic purposes, you have to cite whatever you use in the same way as you would do with books. Only that internet sources are usually less relyable.