"Headphones" = the big ones that go over your head and clamp onto your ears. There are two types, on ear, which have the pads on your ears, and over ear, which have bigger cups and don't put pressure on your earlobes. Generally speaking the latter are more comfortable, but they're bulky. This type of headphone generally has more bass and a fuller sound. There's also "open" and "closed" types depending on if the back is open to outside air or closed. Open can have a more airy sound and wider sound stage but leak sound (other people around you can hear), where as closed are more isolating.
Earphones = generally refers to earbud types, the kind that just sit on your earlobe but don't push into your ear canal. These are less isolating to outside sound and don't typically have the same level of bass or fullness in sound as headphones or even IEMs. Generally these are pretty comfortable for long listening sessions but largely depends on the design. More grab & go type headphones, the ubiquitous apple airpods (non-pro) are a good example of this.
IEMs = In Ear Monitors, this type of earphone has a tip that goes into your ear canal. Usually the tips are changeable and can wear out. Usually made of rubber, silicone or foam. Depending on the tip IEMs can be quite isolating so you can focus on the sound. The sound of this type of headphone can vary widely depending on design. IEMs come in two driver styles, Dynamic Driver and Balanced Armature. DDs are typically more full sounding with more bass, while BA drivers are detailed sounding but may not have as punchy or deep bass. The latter is typically overcome by adding more BA's to each earbud. The more expensive ones can have 3, 4 or even 5 BA drivers in each earbud.
I just updated to this 147 version and the compact mode is very similar to wefwef and memmy. Only difference is the community name is on top of the post title where on wefwef the community is to the left of the post statistics on the bottom.