A considered decision is one that you have carefully thought about for a long time. Choosing what college you will attend should be a considered decision.
To consider something is a verb that means to observe it, or to look at it from many angles. If something has been considered, it has been carefully evaluated. You might have a considered response to an invitation to the prom, weighing the pros and cons of the prom date. A considered opinion is one that you have reached over time.