Figure 1. Source. Good image from Wikimedia Commons. In this one we can see how the blades are put vertically, and you get several of them per horizontal rack space, instead of a tipical single rack server going all the way horizontally.
Video 1.
HP Blade Server by Brian Kirsch (2013)
. Source.
In the video we can see that it contains RAM, disk storage, we are told about two CPUs, and networking interfaces, so it is a complete computer on its own. He also explains that unlike typical rack servers, each blade unit does not have its own coolers and power supply related hardware, which goes instead on the chassis.