Assertion: A style of Communication
Ever find that you end up saying "yes" to a colleague who wants to hand over some of their work to you and rush of on a weekend trip with family; leading to you spending a weekend in office...

...Or saying "yes" for a relative's lunch when all you wanted was a lazy Sunday at home with your family...

...Or going to see a movie you do not really care for with friends.

Do you wish that you could say "no" more often without fear of offending or hurting anyone?

If the answer to the above is "yes"...you probably need to learn some skills of Assertion.
The good news is that skills can be learnt and you too can become Assertive person. 

What is Assertion?Assertion is, in fact, a communication style. It is a style in which you can communicate your needs, wants and feelings to other people without hurting them, or misusing them or manipulating them.
Being assertive would mean that you could behave and communicate such that you could come away from situations without continually feeling bad about yourself, without feeling compromised. Acting assertively will allow you to feel self-confident and will generally gain you the respect of your peers and friends.

The Assertive communications posture says: "I count and you do too."

Alternate styles of expressing oneself are:

Aggressive Style
Aggressiveness involves expressing thoughts, feelings, and beliefs in a way that is inappropriate, threatening and punishing. The aim is to get your own way no matter what.

The Aggressive communications posture says: "I count but you don't count."

Passive Style
It permits others to violate our rights and shows a lack of respect for our own needs while communicating a message of inferiority to others. The nonassertive person decides that his or her own needs are secondary and opts to be a victim.

The Passive communications posture says: "You count, but I don't."

Passive Aggressive Style
The passive aggressive person uses nonverbal behavior to express anger or resentment that they can't express verbally. They repress their feelings and end up showing anger and resentment is ways such as sulking.They rarely mean what they say or say what they mean. Not giving up work in time after having agreed to do so is an example.

The passive aggressive communications posture says: "I count, you don't count, but I am not going to tell you that."

Learning Assertiveness or Assertiveness Training does not concern itself with the causes of the problem but rather with the development of appropriate skills to cope with them.