A Proposition player (Often called Prop, Poker Prop, or Prop Player) is a player paid by the house to start new games and to keep existing games going. Props play with their own money and keep all winnings and likewise are responsible for any losses. The pay from the house/employer comes weekly and pay is generally calculated at 100%+ rakeback or on a per hand rate.
Yes, you can become an online prop player. 75% of new poker rooms use props in order to get their sites started. Some of the larger sites continue to use props but most large sites suspend their prop program once their website has reached a steady mass of player traffic.