Yep that's right! It's an Arduino-like microcontroller development board, but with built-in Wi-Fi and an open source cloud back-end

So does it basically a wifi interface for REST commands and custom functions? (for devices)

Yep, that's pretty much exactly what it does, just tied into their ecosystem instead of being standalone/DIYish like an Arduino. You see people above comparing ElectricImp because it's very similar - simple way to get REST or simple net connectivity for a thing.

Is the device capable of pushing information out too? Or do I need to ping it at regular intervals to get an update or status change in something?

It just seems like it would be great for push tasks, but what about when I want it to send notification or device info.

(I've put together R Pis for exactly this type of thing, and having an entire OS for it is a little overkill)

Very easy to publish an event stream from your code, and to pipe those wherever :) http://docs.spark.io/firmware/#spark-publish

Think of it as a postage-sized wifi router.

With a cloud back-end, and controllable from any Internet-connected device.