One or more unions meeting with representatives from an organization to negotiate labor contracts.
Collective bargaining is made possible through the employer recognition of a trade union - betwee them they will decide on the scope of the negotiations. Most often, collective bargaining is enacted voluntarily - good employers understand there are many benefits to voluntary union recognition. The ability to resonably negotiate terms of a contract and on pay for all represented workers.
If you are a member of a union and your employer recognizes this union for collective bargaining then any improvements in the conditions of your work should be automatically incorporated into your contract. Consequently, the best way to obtain better wages and terms of work is through powerful and effective unionization.