"Prop" business models vary somewhat. In the old days, a "prop" firm was primarily focused on hiring employees to engage in trading practices for the firm, do what the firm said, receive a basic compensation and perhaps a percentage of the profits generated. With the advances in technology and program trading, most of these firms replaced the "traders" with computers.
On the exchange trading floors (historically) another business model took place (one which we at Bright Trading engage in), wherein an independent trader would buy a membership on the exchange (seat), place some money with a Clearing firm (who acted to clear trades and be the "bank"), and then the trader would essentially use the Clearing firm's money to trade with (most often more than a $million, often much more), and the trader would keep the profits from their own trading, and, of course, be responsible for any losses incurred. The Clearing firm would charge for this use of capital and services in the form of commissions and possible interest or risk fees.
My brother and I bought memberships, placed money with SLK (now Goldmas Sachs), and pursued our trading careers. We were very fortunate and did quite well. We were able to negotiate with Clearing firm for lower fees by adding more traders, thus increasing the overall trading volume...making for "economies of scale". We formed BT in 1992 with 2 people, and by the mid to late nineties had hundreds of traders who essentially used our capital to trade, kept all their profits, and benefited from the same economies of scale. Since we had the "big" money put up with Goldman, we were (are) responsible for all the trading money and any possible losses.
Now, our traders don't have to buy expensive memberships, and can enjoy the same benefits as exchange members, keep all their profits and use $millions of our money to trade with (with, obviously, some risk control).
There are only a handful of broker dealer exchange member firms who engage in this practice these days. There are many "Introducing broker" type firms with little or no capital to protect the traders, and there a a few "sub-llc" type firms who are not broker dealers, thus offer little or no protection to their traders (be careful, as always, do the due diligence, check balance sheets, etc., as you would with any business partner).
This model allows our traders to engage in trading strategies that actually work well, much like the strategies used by Lehman, Goldman, etc. (Pairs trading, market making, Mergers, and other simple, lower risk, higher reward, but capital intensive techniques).
Just a quick overview, hope this helps, feel free to call with any additional questions (702.739.1393 mid trading day is usually good).
Don