Imagine you're in line at your local coffee shop, and after a long wait, the barista finally takes your order. Moments go by and he hands you the wrong order. You ordered and paid for a large iced mocha, and you've been handed a small cup of hot coffee. How do you react?
(1) Passive. "Uhh... okay, thanks." You don't mutter anything else and just take what's given to you, and move along. Never mind that it's blistering hot outside and that you really wanted a cold beverage.
(2) Aggressive. "Are you an idiot or something? I ordered an iced mocha, and not this stupid cup of stupid coffee. I want to talk to your supervisor. I know who owns this place and this is not acceptable. This is ridiculous!" The mid-morning coffee house gaggle comes to a stop as everyone notices your outburst.
(3) Passive-aggressive. "You know what? I ordered an iced mocha, and not this drip coffee, but seeing that you're so inattentive and uneducated, I guess I'm not shocked you screwed up such a simple order. Thanks anyway, and I'm just not coming back here again." You walk away without giving them a chance to get your order right. You spoke with such resolve that the line behind you couldn't help but feel bad for the barista you just publicly humiliated.
(4) Assertive. "Hey buddy, I know you're really busy, but I actually ordered an iced mocha, so it would be super if you guys can prepare that order while I step aside and wait for it. These things happen, so I understand. No rush. Thanks."
Although the examples above are extremes, I hope it still gives you an idea of how each differentiates from the next. The best way to communicate with anyone, including yourself, is through the assertive fashion of communication. Get your point across tactfully and professionally. Being overly passive will only make you feel resentful over time. Being aggressive is just no way to treat people, and being passive-aggressive undermines everyone, including yourself. Becoming aware of how you communicate right now will help you tweak yourself to becoming a better communicator. Hopefully this has given you something to think about.