Yakuake is a pretty simple tool. The first time you run Yakuake during a KDE session, it pops up a small dialog saying "Application Successfully started! Press Alt+` to use it..." and then it disappears into the background.
If you use Konsole, Yakuake can take its settings -- background, font, schema, history, line spacing, transparency, and so forth -- from Konsole, or not, as you prefer. Like Konsole, Yakuake features tabs, so it's possible to have multiple consoles running in a single instance of Yakuake.
You can also configure the amount of space consumed by Yakuake, and where it appears onscreen. For example, if you want a lot of terminal space, you can set Yakuake to use 100% of your screen's height and width, or you can scale that down to as little as 10%, though that doesn't leave much room. By default, Yakuake appears in the middle of the screen, but you can set it to appear to the right or left. In short, it's a very flexible application.
If you exit KDE while Yakuake is still running in the background, it'll start up automatically when you log back into KDE.
Yakuake is a very simple application, but it's also very useful. I use it all the time when I want quick access to a terminal to run one or two commands, such as doing an
apt-get update; apt-get upgrade, checking a man page to find the proper syntax for a command, or moving a few files around.