Also known as the Urgent-Important Matrix, this is a very simple tool to use and to understand. You can create your own variant but here's how it works.
First, you list your tasks, projects... Then you will categorize them by using 2 dimensions:
This will allow you to categorize your tasks/ideas/projects in a matrix that looks like that:
Then it's pretty simple :
At comet, we used it a bit differently. We used it to classify and rank our marketing project:
We added two dimensions to the Matrix: complexity and a time estimation. It allowed us to better classify our projects and also to tackle fast and easy projects first in orderto have more time to work on more complex ones.