In its youth it was criticised for not spidering the web as the search engines did. Now it's the most frequently visited site on the net. The difference between Yahoo and the search engines is that real people add sites to Yahoo. Unlike search engines, you cannot just add a new web site or web page to Yahoo. A real live human being has to look at the page and approve it first.
Human approval is both great and awful.
It's awful because not every page gets listed. Yahoo carries a much larger percent of relavent sites than the search engines do, so that when you search for 'Transformer Toys', although you get fewer results, they are usually more relavent. Also, since Yahoo is a directory, you can find your way into the catagories that you are interested in. There you will find landscaping, house plants, etc. On the other hand, you can still use Yahoo much like you use a search engine, by entering key words to search for.