I know this question is from a while ago, but just wanted to clarify -
The above post is right - this category of disorder includes psychological/anxiety-related conditions that cause physical symptoms. The key is that the physical symptom doesn't have an obvious, biological cause - if it does, then it won't fit into this category. This includes conversion disorders, which are serious losses of function (paralysis, for example) that are thought to result from trauma or anxiety. They usually go away eventually, but can be very hard to treat at first, especially since the patient is often unfairly accused of faking / being "hysterical" or otherwise written off.
When they mention "related disorders," I think they're referring to things like body dysmorphic disorder - something that doesn't necessarily cause "symptoms" but is still in this category in the DSM. That one is especially relevant, as (if I remember correctly) it's significantly more common than many of the others. Basically, BDD is an extreme and debilitating focus on physical appearance, usually specific to one aspect (like weight), to the point where a flaw is seen where one doesn't exist.
"Somatoform disorder" is technically the old name, DSM-5 reclassified them as "somatic symptom disorders," like OP said.