Usually it's here where the formerly uninitiated says, "Oh, I get it now!" If that's you, congratulations and welcome to the club. To further your understanding, here are a few more differentiators to help you choose assertiveness over aggressiveness:

Aggressive conduct: Glares or stares at others
Assertive conduct: Makes friendly, considerate eye contact

Aggressive conduct: Intimidates others with body language
Assertive conduct: Shows confident body language that matches the message

Aggressive conduct: Has an air of inflexibly-"my way or the highway"
Assertive conduct: States one's needs, but genuinely considers other perspectives

Aggressive conduct: Strives to control others
Assertive conduct: Strives to listen to and work with others

Aggressive conduct: Considers other's perspectives only when demanded to do so
Assertive conduct: Considers other's perspectives without needing to be asked

Aggressive conduct: Values one's self more than others
Assertive conduct: Values self as an equal to others

Aggressive conduct: Will intimidate or even hurt others to avoid being hurt
Assertive conduct: Tries to hurt no one (including self)

Aggressive conduct: Reaches goals usually on the backs of others
Assertive conduct: Strives to reach goals, and help others reach their goals, too