I think you need to be more specific with your question.
There are good explanations on Ethereum's webpage, have you visited and read through it?
https://www.ethereum.org/Ethereum is a blockhain app platform. They provide a system for plugging in or building your project on to the blockchain technology. The benefit is that it eliminates the need for some one/organization/government/company to build their own blockchain upon which to build their project.