What is GlassFish?

Question

While developing a servlet in NetBeans, I saw something called GlassFish Server at many places such as in New Project dialog. I want to know that what is Glassfish Server and what are its uses?

Answer 1

GlassFish is an open-source application server project started by Sun Microsystems for the Java EE platform and now sponsored by Oracle Corporation. The supported version is called Oracle GlassFish Server. 
From:Wikipedia

Answer 2

Does it mean that Glassfish is like runtime environment for web apps developed using Java technology?