Google Scholar is a search engine for scientific publications, case law, and patents. Unlike PubMed it includes an estimated citation count and is more tolerant to search terms like misspelled author names and titles.
Pro and Con
+ more tolerant than PubMed to bad search terms and search terms including issue and pages numbers
+ searches patents in addition to published articles
- command line search is less powerful than in PubMed
- Google hides how articles are ranked, biasing subsequent reading and citing
- fewer hits are peer reviewed and are thus of slightly lower average quality compared to PubMed
Ranking of articles
From what one can guess, articles are ranked mostly by citations weighted by who cites, i.e. 1 citation from PLoS Biology is worth more than 1 citations from the Antarctic Journal of Aquaculture. Because of this ancient articles are often among the top hits. A search for cancer returns a 1981 papers first as of March 2015. It is therefore useful to use the year restrictions to get to the recent papers. Unfortunately, there no quick way to do this with search tags. You have to click the interface.