Alright sure, I'll explain it all:
The guard/attack system is divided into three directions: Top, left and right.
The three directions shown in white/gray on your character are your guard.
White for where your guard currently is, gray for the other two.
The white direction shown on your enemy is telling you where his guard currently is.
When the enemy attacks you, it shows where the attack is coming from with the red indicator. If your guard matches his attack at the time the attack hits you, you block it. So it doesn't matter if your guard was already in that direction or if you switched it at the last moment, if your guard and the attack match as it hits,you block it.
Now most classes always have their guard up in one direction, but the assassin classes don't. They have a timing based guard where you push in a direction and for about 1.5second there will be a guard there. If they guard at the last moment, they actually deflect the enemies attack,something only assassins can do.
What's also important is that parrying exists. See if you block a light attack, it just gets negated. If you block a heavy attack, you take some damage through your block (this damage can not kill you). Now, when you see an enemy attack, right before their attack hits,their red attack indicator will blink. If you press your heavy attack button in that moment, you parry their attack. This negates all damage, throws them off of their guard for a moment AND the attack the enemy used costs him twice the normal amount of stamina. If you miss the timing of the parry however, his attack hits you unblocked.
Positioning is important when it comes to range of your attacks and dodging.
Heavy attacks are slow, light attacks fast. Heavier classes are a bit slower in general, yes.
Friendly fire is ALWAYS on. You hurt friendlies with every attack and you interrupt their movement/attacks with yours.
The lightning+exclamation mark means it is an unblockable attack. It can only be dodged or parried.
The shield icon means the enemy is throwing a guardbreak at you. If you press your guardbreak at the time the shield shows up, you rebuff his guardbreak.
I you have any other questions,ask away
For fighting multiple people, check the OP please, I explained it there!