A group of programmers who make a popular version of Linux have decided to make their version of Linux start with a new program when the computer boots. Instead of using a program from the 80's called "System V init scripts", they have decided to switch to a new program called "systemd." Both programs have the major responsibility of starting the rest of the programs that make up Linux.

I found this explanation of systemd interesting (though more suitable for a wannabe hacker than a 5-year-old (though I suppose the two sets may not be disjoint)): http://0pointer.de/blog/projects/systemd.html