These are good sources to use when you are searching for a very broad topic or when you want a general idea of the resources available on a subject. These are more like databases, manipulated by their producer to make them more usable. They often are searchable by reviewed or rated sites, which helps to eliminate irrelevant results sometimes retrieved by the larger keyword search engines. These include groups of search engines which can be searched either simultaneously or using one search interface. They have the potential to be slow.
Most search engines have an introduction on engine use or a help button.
If you spend time in an engine, learn how to use it efficiently. In some engines you can specify how broad you want the search to be. Search engine language syntax. Like most software, engines change over time to incorporate new features or make the interface easier to understand. Expect that to occur.