Summary
Hold Back to block all high, mid, and Jumping Attacks, as well as Overheads. Hold Defensive Crouch to Block mid and low attacks.
Don't Get Hit
Blocking is your main means to defense. Although it sounds completely obvious, learning how to Block is one of the most fundamental skills in Fighting Games, and it'll never be more important than in Marvel Vs. Capcom 3. With the amount of angles you can be attacked from, Blocking is truly a skill in and of itself.
Blocking will obviously allow you to avoid taking the damage from your opponent's attacks. Once you Block an attack, you are rendered "stuck" in Block Stun and will freeze in place for half a second and then start sliding back a fixed distance, depending on the strength of the move you blocked. If you are in the corner, then the opponent will be pushed away from you as opposed to you sliding back. But that only happens if the opponent is on the ground. If they are in the air, they will not be pushed away from you while you are blocking.
Block Stun
If another attack connects on your character before you end your Block Stun, you will again be put into Block Stun and forced to Block again. In fact, when a second move connects while you are in Block Stun, you can even let go of the controller altogether and still block the next attack. So for example, if Iron Man performs his Unibeam, once you Block the first hit of it, you can let go of the controller because you will automatically block every other hit from the beam.
This is what is known as a Block String and also known as Absolute Guard. It's a sequence of attacks that, once you Block the first hit, you are trapped in Block Stun for the entirety of the sequence. It's basically a "combo" against someone who is blocking.
However, if you are blocking in the wrong direction, even during a Block String, you will still be hit. So, let's say the enemy does a Jumping Hard Attack from Spider Man and you high Block it. If he lands and does a Crouching Light Attack, which hits low, while you are still in your Block Stun and you don't switch to crouching, you will get hit. You still have to adjust high or low for blocking, even during Block Stun.
Strategy Corner
When it comes to breaking the guard of the opponent when they are on the ground, you really have to take advantage of the fact that, while on the ground, the opponent has four different directions from which they can be attacked. Between 
or 
and left or right, you are given four angles in which to strike: on the left side, on the left side, on the right side, and on the right side. By utilizing your moves properly, coupled with Assists, you can make it a nightmare for your opponent to safely block all four vectors of attack.
For example, if you activate Kitty's Helper with Felicia, you can have her helper kitty attack from the sky and her pounces must be blocked high. Meanwhile, you can attack with an Assist from the 
and then start using either your Cat Walk or Medium or Hard Cat Spikes to jump over the opponent. You can even just continue attacking the opponent with attacks until the helper kitty lands on the head of the opponent, creating an unblockable.
Another example is with Storm. She not only has Air Dashes for Triangle Jumping, but she can 
her regular jump instantly into her floating ability which cancels her regular jump halfway up, allowing her to perform an air attack that must be blocked almost instantly. However, she can also choose to Air Dash down and land with a Crouching Light Attack, which must be blocked low. She can even jump forward and Air Dash 
behind you and attack you from the other side, which is particularly effective after calling an Assist to attack from the original side. So with all of these options, it's really easy for Storm to trip up your blocking while you are on the ground.
These are going to be the hallmark of successful attacking with rushdown characters. If you cannot figure out a way to trip up your opponent's defense, you will have a very hard time in defeating them when they clam up when you get close. There are other ways to break defenders -- chip damage, Throws, etc. -- but confusing them as to which way to block can easily be the most effective. Not every character has the tools to perform such mix-ups, but if you do have them, learn to use them.
Proximity Blocking
Marvel Vs. Capcom 3 also has Proximity Blocking. Your character will only block if the enemy's attack actually touches you. Otherwise, you character will never get stuck in block animation if the opponent's attack whiffs, no matter how close it is to hitting you.