Top search engines shows you all search engines which referred traffic to your site, ranked in order of visits generated, for any given period of time. Popularity is defined in terms of the most visits generated at the top.
The list can be expanded per search engine, meaning that you can see the sub-domains for each search engine. See screenshot below for example.
Use the calendar to select your date range.
The columns displayed, from left to right, are: No., Rank, Search Engine, Graph, Visits, Avg. pv/ v.
Avg. pv/ v. means average pageviews per visit. This is a method of measuring Traffic Quality; in many cases, the better quality the traffic, the more time they will spend on your site. For example if you are trying to decide between purchasing traffic from two separate search engines, and one source delivers traffic which looks at 1-2 pageviews on average, and another looks at 5-7 pages, then there is a clear differentiation.
The Rank column shows you if popularity has changed over time, using green and red arrows.