It has rechargeable battery which is charged via a small USB adapter which plugs into a USB port and then the Pixel can "sit" on that.
It can work with any iPad or iPhone as a dumb stylus, but the magic happens when you power it up and then use it with applications that know about stylus' (there's usually an option is the settings area of the application to enable it).
As it uses Bluetooth it only works with iPhone 5, 5c, 5s, SE, 6, 6 Plus, 6s, 6s Plus, iPad Mini, iPad Mini 2, iPad Mini 3, iPad Mini 4, iPad 4, iPad Air, iPad Air 2 and iPad Pro 12.9 (don't quite know why it won't work with the smaller iPad Pro). Packages that support it are Photshop Sketch, Concepts, Procreate, Illustrator Draw, Autodesk Sketchbook, Astropad, Picture Photo Studio, Goodnotes, Penultimate, Notes Plus, Noteledge Cloud, Noteshelf, Myscrypt Smart Note plus many more (and more being added all the time as Adonis have an open SDK that people can use to integrate the Adonit features into their apps).
In terms of features, the Pixel's nib is pressure sensitive so things like line widths or increased/decreased shading will correspond to the pressure being applied, there are also two short-cut buttons that can be defined within the application (say to change brushes or undo/redo). Supports apps also support palm rejection (you tell it your pen holding style) and that can make a big difference as putting your palm on the screen will normally confuse the app and may apply paint (or whatever's being drawn) where your palm is. This can allow a much more comfortable drawing position without worrying about where your palm is resting.
The only gripe is that if you have a screen protector on your Apple device, it can make it feel a bit squidgy (the screen protector in use has a soft texture, a glass screen protector would fair better).
If you're into drawing or note taking on an Apple device, this could be a useful addition for you.
Amazon stock the Pixel for £79 but it can be had online for £59 if you look (it's also cheaper than the Apple Pencil which is only supported on iPad Pros).