In this tutorial, you'll learn the absolute basics of the embedded world. If you have already experience with embedded systems and/or electronics, this chapter probably doesn't provide anything new to you.
None! This is your entry point into the embedded world!
Wikipedia does a good job defining embedded systems with this opener:
An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electrical system.
So at the end of the day, if you are adding any sort of computation to some object who's main purpose is not being a computer, it's an embedded system.
Some examples of Embedded systems:
An important characteristic that's often required for an embedded system is "real time". This is simply the ability for the system to respond to an input within a hard deadline, Eg. automatic breaks for a car. A general operating system like Linux is not suitable for these applications because it uses time sharing when scheduling tasks/programs, and unreliably responds to important signals.