The main difference between a professional and an amateur poker player, other than poker being the sole income for the professional, is the mindset of the individual, from controlling emotion while in a session, to understanding the long-term expected value of playing a hand, versus winning or losing any one particular pot.
A professional evaluates and re-evaluates his opponents and their likely holdings each and every time, along with what that opponent’s tendencies are and how they may change throughout a session. An amateur merely plays his own hand from start to finish based on his perceived actual hand strength for that given board.
A professional always knows and understands the long-term expected value of the hands he plays, given all the variables he has taken into account based on his opponent. For a large percentage of the time an amateur will simply play the cards he feels are best for that hand, without taking into account the full effect of whether or not that hand will be a winning or a losing hand, and does not take into account the variables of his opponent’s tendencies and the types of hands played.