It's available standalone (ie: as a rebadged Eclipse), or as an Eclipse plugin, which you can then use a seperate perspective, or simple as an editor for js/css/etc files within your own perspective.
Eclipse is a great idea and would be a great product if it had sensible & smart people running it, but it doesn't and so it's only a good product.
Anyway, as I mentioned, Eclipse revolves around a workspace (a set of projects) and perspectives (a set of panels).
Most Eclipse plugins provide an entire perspective dedicated to themselves, with all the appropriate views (panels) around the central editing area.
Each file extension is associated with a default editor type, and when you open them you can optionally have the perspective jump to the appropriate one for the type of file you're editing.
Or, you can simply customise the perspective and have a single set of views which doesn't change but has all that you need in tabbed groups which you can minimise/activate as necessary.
The main benefit of doing the rebadged Eclipse thing is that it has its own install and own set of plugins, which helps to avoid conflicts arising.