People get arrested, sometimes people just like you. You think it can't happen - you're an upstanding citizen who gets up early, eats sensibly, and buys more Girl Scout cookies than you really want. Then you choke out your neighbor in a property line dispute. It happens.
So then what? Well, much of that depends on whether your charge is a misdemeanor or a felony. A misdemeanor is a criminal charge punishable by up to a year in jail or probation. The fines for a misdemeanor are relatively modest. A felony is a charge carrying a sentence of a year or more, and more severe fines, probationary periods, and other undesirable attachments, like death and stuff.
Of course those are just within the formal criminal justice system. The difference between a misdemeanor and a felony conviction is nearly as significant in other aspects of life such as the freedom to exercise certain rights, and gain employment. These can include voting, gun possession, and disclosure on a job applications or security clearance updates.
How are crimes classified as misdemeanors or felonies? Nobody knows. Okay, actually, the legislature decides it using a methodology called "whatever we decide."