Selenium is a software testing framework developed a decade back in 2004 by Sir Jason Huggins at Thoughtworks. It is a highly flexible software .The USP of Selenium is, it supports testing on multiple browser platforms. Released under the Apache 2.0 license, It is an open-source software, that can be downloaded & used without incurring any expenditure.
Selenium is a set of different software tool, each tool having a unique approach to support test automation. Most Selenium QA Engineers thus adopt either one or two popular tools that meet the needs of most of their projects. However it is advisable to learn all the tools to acquire greater expertise with respect to different test automation problems.