Mmkay this is actually in my domain since I'm autistically obsessed with abstract impressionism.
So impressionism is trying to capture the "feel" of something, it can be realistic, cartoonish, cubist, anything. It's more involved with color theory and line weight than an actual image from what I remember. A good example are cartoonists that draw people on the streets.
However, the most commonly defined version is art that is focused on lighting, strange angles, and delicate linework.
Pure impressionism is often described as "dreamlike" in execution.
Many people liked the look of it, and decided to incorperate it into other genres. (A lot of art is mixing things that really shouldn't be mixed to see what happens. I blame the lead paint)
Realism is a specific subcatagory with a bit of a range. Archer would be considered realistic because they do actually look like certain adults, or give off the impression that they do, all the way to photo-like drawings. This means impressionism CAN fall under the catagory of realism, but it's still technically its' own thing on both sides.
Ain't art fun?
Oh! And when I refer to abstract expressionism, here's what I mean:

(Fun fact, I'm left handed. I did this on my phone with my right to see if I could)

So the above is not actually a dragon, there's no head to it, it's only tails. The eye can be seen as a wing, and some fire can be seen as flowers.
The expressionism is in the shading and colors, while the small, fine lines of paint are more impressionistic. It LOOKS like a dragon, which is kinda what impressionism is, wven if you just slap other shit on top of it. By itself, the dragon one wouldn't be impressionistic, the shading would be considered far too heavy. The sunset could get away with being impressionistic instead of abstract expressionism though.
Edit: fair warning to new people, I doublepost a lot in this thread, I'm a phonefag and also incredibly lazy.
I do my best not to in other threads, but I couldn't give less of a fuck in Emily's.
Editedit: ANOTHER fun fact, that dragon was started with 2 thick, black spray paint lines.

I started at the firey/flowery end and worked from there.