Is it a bet on price fluctuations and against the house?

External question — Personal Finance Stack Exchange Author: Savann Original post: https://money.stackexchange.com/questions/30924 License: CC BY-SA 3.0 — https://creativecommons.org/licenses/by-sa/3.0/ Adaptation: HTML converted to plain text; contact email addresses removed. When one trades Forex, CFDs, metals, or stock indexes with online brokerage firms, does it mean we bet on the price fluctuations without actually owning the underlying assets and is it a bet against the house (against those brokerage firms)?