Too broad for me to provide a concise response. In what regard? Who are the parties in question? Boss- to- employees, I'm always assertive and need to be. If I'm being aggressive then something's wrong on my end. My point can and should be effectively communicated through clear language, word choice and tone.
Re: sales, we have goals to meet, and sometimes aggressive strategies for up selling, being very direct with client businesses, etc can mean the difference between them going with our brand over a competitor.
At work, I think the line between assertive and aggressive is drawn at making it personal. I always try to avoid the words "you" and "I" or any variation of that - making the conversation about a thing, rather than the person. I think as soon as someone hears the word "you" directed at them, they take it personally. Not using it fosters more cooperation, in my opinion.
Assertive: All vouchers should be completed by Friday at 11:00.
Aggressive: You need to have your vouchers on my desk by Friday at 11:00
Assertive: The size of the picture on page 4 should be a little larger.
Aggressive: You should have made the picture on page 4 larger.
In terms of behavior:
It's all about body language, posture, tone, volume, and voice inflection.