Before I got into philosophy, I used to be an avid student of politics. The current state of politics where I'm from could make that seem like a major leap, but regardless it was part of what made me who I am and how I got here. I find it interesting when I hear people like my buddy 
I feel that these two are mistaken by many, but once we understand assertive behavior the differences become very clear. Aggression is the opposite of passive behavior, but there are ways to manipulate that as well through passive-aggressive behaviors and actions. People using aggressive tactics usually disregard any concerns of others in their pursuits and view things as competitive. I tend to look at life now as a collaborative effort and value the contributions of others, but I realize not everyone sees things that way and I can usually still work well with both passive and aggressive people depending on the task at hand.
I would say the simplest way to understand passive behaviors is "letting things happen and being at the mercy of others" and aggressive as "making things happen with no regard for others' well-being." If we view those two as opposite ends of the same spectrum, I would say assertive behavior lands squarely in the middle. Assertive behavior recognizes the needs and desires of others and defends its' own interest. We seem to live in a predominately "dominating society" where there always has to be a winner and loser, but if we shift that view towards cooperation and collaboration, we can learn to be assertive and defend our own well-being without being aggressive towards anyone else. This often leads to opportunities to do better together through cooperation.
Assertive behaviors generally require a higher level of self awareness as we must be willing to be self reflective of our own behaviors and determine when they are being harmful or infringing on the rights of others. If we are aware enough of our own actions and intentions and respect the rights of others then we naturally treat them as equals. I feel that the blurry area is when it comes to firmly upholding and defending our own rights that people mistake assertive behaviors as aggressive in nature. Aggressive individuals will often need to talk over or put down the person they are speaking with and take the same stance in business. Assertive individuals on the other hand will hear and respect the other while firmly maintaining their own beliefs or interests and looking for ways to either work together or part in peace. I think it can be summed up best as, "Do no harm and take no shit."
It's at this point that we can see the ways that assertive behavior differs greatly from passive behavior as well. Where a passive individual may be willing to let an aggressive individual dominate them or infringe upon their own rights, an assertive person will not allow that to happen willingly. Assertive individuals do not use violence as a form of intimidation or control, but will use it to protect themselves. This exact issue is why I can't consider myself truly pacifist. I would have no problem killing someone in self defense or forcefully stopping someone from stealing from me, where a passive or purely pacifist individual would be more willing to let something like that happen and just be okay making it out alive. Personally I don't think oppression is ever acceptable and I don't aim to dominate anyone, but I do believe it's my responsibility to enforce those limits for myself if it comes down to it. Namaste.