They are sometimes called web directories. There are several major guides or directories that you are most likely to find useful, and they are listed here. For some kinds of searches, you may want to use only a search engine. But when you're getting started on most topics that you don't know a whole lot about yet, the right subject guide can be a real goldmine as a starting point. Note what kind of site you're looking at when you start on the subject guide.
Subject guides vary quite a bit in what they offer, and the divergence seems to be increasing.
Sometime there is worthwhile material in their subject guides, but you need to keep in mind that the site managers are most concerned with providing whatever will bring in the most users, and therefore lean heavily away from the academic or weighty and more toward the fizzy popular stuff. Subject guides on sites without ads are likely to be more academically serious and select more serious material. However, there still are some good subject guides on the more commercial sites, and you should explore some of them to locate material that may be most valuable for you. Decide whether you want to start with a general or a specific subject guide. So look carefully at the general subject guides that are included here, your first few forays in this arena.