Technically, a poker run is not a race but sometimes the big horsepower players run fast to the card stops.
Participants in a poker run travel to designated 'card stops' where playing cards are handed out at each one. At the end of the day best hand wins. Rules vary depending on the event- sometimes multiple cards are given out at different stops. a stop can be a boat, a dock or a restaurant- any place or thing that is accessible to the boats. Usually, one of the card stops is also the lunch stop.
Here's a link to a slideshow from a run in Sarasota on Labor Day.