Depicted above: how not to do it
Getting hit is a part of fighting: you can't expect to participate in a pugilistic engagement without accepting that you're going to get hit. But at the same time, the person who gets hit less usually wins, so one should strive to reduce the amount of strikes they receive to the bare minimum. There's 4 ways to do this, and they're not all created equal.
Understanding the hierarchy of defense will change the way you look a fistfight, and probably inspire you to re-prioritize what you spend time drilling.
Starting from the bottom up, the 4 methods of defense are blocking, parrying, head movement, and footwork.
1) Blocking is simply the act of putting a barrier between the oncoming strike and your personage. That barrier is usually going to be your forearm or gloves. This is the easiest type of defense to perform, but despite being way better than getting hit, it's also the worst.
When you block your opponent's strike, they can rebound off the impact and build momentum for a combination -- much like striking a bag. So if your only means of defense is covering up, your adversary can keep striking you until they run out of steam. Or until you deter them with a strike of your own.
Unfortunately, that can be difficult since blocking requires the use of at least 3/4 limbs. You'll need a minimum of one arm to serve as the shield, and both feet must be planted to remain on balance when absorbing a blow.
Because blocking uses up most of your resources AND does nothing to disrupt your opponent, it's best used as your last line of defense.
2) Parrying a blow is an upgrade from blocking since it can disrupt your opponent's rhythm and even compromise their balance, giving you an opportunity to counter. Parrying requires the use of only one arm, leaving you with more freedom to move and more options for counterattacking than blocking does.
3) Head movement is comparatively difficult to implement correctly, but when you do, oh boy does it pay off! Making your opponent miss is highly disruptive to their flow and balance. Better yet, properly moving your head will set you up for a devastating counter-strike. (In fact, most counter punch knockouts in boxing are set up by head movement.)
Aside from being highly disruptive to your opponent, head movement is minimally disruptive to yourself, in that it ties up less limbs than blocking or parrying. While the leg that you lean most heavily on will be spoken for, the other three are available as instruments of retaliation!
4) Footwork, while certainly the most difficult to master, is indisputably the king of defense. Your opponent can't hit you if you're never in front of him or within his teach. A mastery of distance and angles will rob your adversaries of the opportunity to get their offense going. Before he can even worry about throwing a strike, he must first line himself up with his moving target.
Even when that happens, honed footwork will allow you to evade on the fly, and re-position yourself for devastating counter attacks. Defensive footwork is the most disruptive to your opponent's balance, utilizes the least of your own resources (limbs) to perform, and sets up the most varied and damaging options for returning fire.
Perhaps what's most interesting about the hierarchy of defense is that efficacy and difficulty go hand in hand. Blocking is the least effective defense, but is the easiest to perform; footwork is the most difficult to use effectively, yet yields the highest payoff.
Obviously, a skilled fighter will use all of these tools to protect themselves from harm, and each fighter will garner their own preferences with experience. Even so, having this overview can help contextualize the game and hopefully convince you to do some more footwork drills.
Good luck not getting punched in the face!