Fare dodging comes in many forms, and as well as the usual stereotypes about chavs, kids and yobbos, also - in my view - includes:
a) Commuters who buy Open Returns and then, when the "Any tickets from X" call is made do not hand over their tickets for inspection as they know the ticket will get stamped and they will not be able to use it again. By keeping quiet their ticket will not get checked and they will use it again.
b) People of any description who pretend to be asleep or stare out of the window hoping you won't trouble them, and when you do they often do not have a ticket. More often that not, in my experience, these are regular business travellers.
c) People who suddenly need to go to the toilet in a hurry when the inspector comes. A few weeks ago whilst doing a call over ticket check between two major stations on a busy train I noted a male suddenly leap out of his seat and tried to get past me down the aisle towards the toilet. As I did not recognise the male and was pretty sure I hadn't seen his ticket already, I asked to see his ticket. He told me, in front of the whole carriage, that he was desperate for the toilet, if he didn't go now he would wet himself and he didn't have time to find his ticket before going to the loo. I noted the toilet was engaged and advised the male that he had plenty of time to show me his ticket as the toilet was engaged anyway. After the "ticket dance", much patting of pockets and rummaging through his wallet 3 or 4 times, it came to light he didn't have a ticket at all. I duly sold him one and he returned to his seat. Now, feeling in a slightly sarcastic mood, I then said "The toilet's vacant now sir, I thought you were in a desperate rush to get there, or has the urge suddenly gone?". The male just went red, and the rest of the commuters were sat there in fits of laughter. This male was a businessman in a smart suit.
d) People who buy discounted tickets and are unable to produce a railcard.
People who get on to buy on board, and will do so without hassle, I do not count as fare dodgers. Those that deliberately seek to avoid payment of the correct fare by pretending to be asleep, looking out of the window, ignoring you, buying railcard tickets they are not entitled to use etc etc are fare dodgers in my opinion.