I've tried to break down the different IoT categories and identify generic characteristics that apply to them. There are of course hundreds of specific SoCs to choose from but they all typically fall under one of the five categories I've mentioned.

High-density compute nodes are a specific type of processors that have manycore CPUs. The reason they are called high-density is because they pack tens or hundreds of small CPU cores onto a single die. These types of small cores don't provide massive single-threaded performance by themselves but when added together, they can be quite compelling. This is a different approach to having a few but very powerful cores such as Intel Xeons on an SoC. An analogy would be a flock of starlings versus an eagle. Moreover, many CPU designers today have abandoned the practice of scaling in frequency to achieve performance and are focused more and more on performance per watt (hence the term green computing). This is because the costs associated with powering and cooling a data center are rapidly rising.