I have been reading about Node.js. But I don't understand why is it claimed to be powerful. In which scenarios does Node.js shine? and can it be used in general purpose web applications?
Node.js is event based in comparison to most other systems, used to build web applications. This lead to it being very controversial (some are very fond of it, others hate it…). Aside of that controversy it can very well be used to build all kinds of web applications, and even big companies have done that so far (LinkedIn IIRC).
Felix, (former) node.js core contributor, wrote this guide, which shows quite well what node can/should (not) be used for.