Plant Bugs are really what these are called; it's not just a generic term for insects that crawl around on plants.
There are many, many different kinds, going by various names; shield
bugs, stink bugs or tarnished plant bugs among them. These are all true
bugs, and in case you wanted more information, related to cockroaches.
They have a hard shell, in this distinctive shape; some are long and slender, others like this one, are wide and flat. The white spot on this one is not just a reflection, it's actually a silvery colored part of the hard shell.
These bugs generally have a long snout, which they use to suck out the juices of plants; in some crops, specialty plant bugs are considered pests, such as the Superb Plant Bug which damages alfalfa.
Mostly, these are not really considered pests, as they don't become too numerous, and are almost all specialists, sticking to one plant or plant family. Many are distinguished only by the plant they feed on, and only a dedicated entomologist could tell them apart.