It's a hard one to describe, even now when I'm grinding the bills off cranks on rocks, sometimes I'll get a bite and I'll ask myself "is that a bite?", and my mind will scream back "YEAH IT"S A BITE, SET THE HOOK!". Fortunately, as mentioned above, if you have a relatively soft tipped rod, it's a bit forgiving and gives the slow folks such as myself a chance to catch that fish.
I always like it when they knock slack in the line, you are cranking away and it feels like someone cut your line - weightless - seems those bites are usually good fish too.