A useful way of looking at assertiveness is to compare it with being aggressive and being passive.
Being Aggressive
Being aggressive means getting your own way at the expense of other people, putting them down, making them feel small, incompetent, foolish, worthless or tricked - manipulating them for your own ends. Being aggressive does not necessarily mean using conflict and anger all of the time, humour can be very aggressive.
Being Passive
Being passive means ignoring your own interests, needs and goals and not being active. It often means putting up with a situation which makes you burn inwardly rather than being honest about what you really think and really feel.
Being Assertive
Being assertive means being honest with yourself and others. It means being able to say what you want, need or feel but not at the expense of other people. It means being self confident, positive and understanding other people's point of view and being able to behave in a rational adult way. It means being able to negotiate and reach workable compromises. Above all, being assertive means having self respect and respect for other people.