Quite simply put, an infraction is the lowest level of crime there is. It's something like a speeding ticket or maybe playing your music too loud or you're having a party and your neighbors complain. That would be an infraction.
There's no jail time imposed and it typically ends up in a fine and you're not arrested. A misdemeanor is something that's a little bit more serious, such as possession of concentrated marijuana or maybe now under Prop 47 possession of cocaine, where you more than likely will be arrested and you might have to spend some time in jail, but the maximum you could go to jail for would be one year and typically the fine is no more than $1,000. And then a felony is the most serious of crimes and includes murder, rape, robbery, sales of drugs, possessions of guns by felons, things of that nature.
And the maximum possible penalty on a felony is death. You can actually be put to death for some serious murders and other crimes.