A search engine simultaneously goes through a large percentage of the web trying to find pages containing the terms you have specified. The search results are then presented with varying degrees of organisation and hopefully you will find something useful. A common experience for the 'naive surfer' is to click the search button on their web browser and subsequently use the existing default engine. In the search box they will then enter a few words without giving it too much thought. Searching the internet is a skill, but thankfully it is one that is easy to master.
Most people choose one, learn how to use it and stick with it.
The disadvantage of this approach is that each engine will produce different results. Limiting yourself to one could potentially result in you missing out on something important. The advice therefore is, if at all possible use at least two to ensure a wider courage. Berkley gives an excellent summary of all the characteristics of each of these engines and I would claim it is possibly the single most useful site on the web. Most university library catalogues are now available on-line.