Systemd is a replacement for the classic init during boot.
When booting most linux systems many items need to be bootstrapped, services started, networks configured, etc. One of the classic ways to do this was init and init scripts. This worked quite well, but performance was sometimes lacking, which was not an issue on classic setups sunch as servers.
However over time the setups diverged and a modern Linux program may need to consider many variations in these scripts.
Red Hat (in the form of the software engineers Lennart Poettering and Kay Sievers) wrote a replacement called systemd. This 'new init' is supposed to be faster and more capable, and hopefully the new joined standard on many Linux distributions.
There has been some complains about the current stability and the complexity of the new, tightly integrated system. While stability is likely to increase over time the system does not adhere to the Unix philosophy, making it a hotly debated issue.
