Wikipedia is an openly accessible - and editable - encyclopedia. The articles highlight dates, events, people, and provide an overview for your topic. This is extremely helpful for gathering keywords and gaining an understanding of your topic. The problem with Wikipedia is that they can be edited by anyone - while mistakes are caught quickly, they can have errors. An alternate is to use encyclopedias from library databases. These articles must go through a rigorous review and editing process before they are published, and they cannot be easily altered, so you know that they are credible.
The database links on the right contain thousands of encyclopedia articles to browse for more information on your topic.
Encyclopedias are bound texts with multiple articles that are focused on a particular theme. For example, an encyclopedia about Dreams might contain articles about boats, but only from the angle of what they mean in a dream; not their full history. Encyclopedias about imperialism are going to provide articles that are focused specifically on countries, people, and events within the context of how they interact with imperialism or colonialism.