What exactly is Hadoop? A webserver, a database?

Can anybody tell me what Hadoop is exactly? I only know that it helps to load balance big data processing if I am right. But what is it exactly? Is Hadoop a database or a webserver? Which environment is required for it? Linux Ubuntu? Does it work with caching? I have absolutely no idea how Hadoop can help me. Can anybody give me a detailed description for it which goes beyond a raw definition?

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