Google - good for taking a very wide view of a topic, using a variety of materials not found in library databases (blogs, some news sources, reports, etc.) Domain searching can target your search results example, "climate change" + :.edu | :.org | :.gov.
Google Scholar - still a huge swath of information, but more focused on scholarly publications (including journal articles, book chapters, conference proceedings, etc.)
Web of Science, Scopus, and JSTOR - academic databases with an explicit focus on scholarly publications, with many options for complex, targeted searching
Specialized databases - some are focused on a subject area (EconLit, America: History and Life, etc.), while others are focused on a type of source (news, primary sources, images, videos, etc.)
Index of articles from scholarly journals spanning the sciences, social sciences, arts, and humanities. Provides cited reference searching capability.
The Web of Science “Core Collection” includes the following:
Arts & Humanities Citation Index (AHCI)
Book Citation Index - Science (BCKI-S)
Book Citation Index - Social Sciences & Humanities (BCKI-SSH)
Current Chemical Reactions (CCR-EXPANDED)
Emerging Sources Citation Index (ESCI)
Index Chemicus (IC)
Social Sciences Citation Index (SSCI)