Chao Phraya Express Boat service has 4 routes, namely Local Line (no flag), Orange Flag Boat, Green Flag Boat and Yellow Flag Boat.
Local Line (no flag boat) stops at every stops
The Orange Flag, Green Flag and Yellow Flag boats stop at selected stops along the Chao Phraya river, with the Yellow line having the least number of stops. Click here for the Chao Phraya express route details
From my personal experience, if you are taking the Orange flag boat, do not buy ticket from this counter at Sathorn pier, they will charge you 40 baht for a one way boat trip and will tell you its the same price on the boat.
Ask the girls wearing the blue vest in the photo and they will tell you tickets can be bought on board the boat at 15 baht.
There are 2 queues to board the Orange flag boat, go to the queue on the right side (without tickets)
Pay the fare onboard the boat, like how the locals do!
More details at the website : Chao Phraya Express boat website