Java is a general purpose high level programming language
Java is a Technology.
Java is an OOP Language.
Java is a Platform.
A Platform is an environment (hardware and software) where we can execute our java program. Java has it's own JRE(Java Runtime Environment) thats why we can say java is itself a platform.
Java is a highly secured language to develop application software.
Java came to the market by taking three editions like:
J2SE(Core Java) is the basic foundation. If you want to expertise in JAVA you have to know Core JAVA First. Core JAVA is specifically used to develop standalone application or desktop application.
J2EE(Adv.Java) is specifically used to develop enterprise application. At industry you will use J2EE widely for developement of application.Now a days JAVA Frameworks(Spring, Hibernate, etc.) are highly demanded at Industry for developing application.
J2ME is specifically used to develop Mobile or device applications.