You would say "different kinds of
windings"
Basically, a string has a
core which is a wire or group of wires that are twisted together. This makes the
core or center of the string. The
winding is another wire or group of wires that is
wound around the core.
So when you say
roundwound strings, that means the winding wire is round wire and will have ridges and is not completely smooth under the fingers. If you say
flatwound strings, then the winding wire is flat and smooth to the fingers.
The core wire on most strings is steel, but the winding wire may be nickel or steel, or sometimes nylon tape.
In general, roundwound strings are brighter and have more sustain. Steel wound is brighter than nickel wound. Flatwound strings usually are less bright and produce more bass and have less sustain.
There are exceptions to these generalizations. There are some flatwounds that are brighter and some roundwounds that are bassier, but it is usually as I have described it.
I don't know about the Olympia strings, but I will say that there can be a lot of difference in the quality and sound of strings. But more expensive doesn't always mean better.
I hope this helps!