Task: A task is something you want to do, but is too complex to achieve in one step. Eventually you may want to build subtasks. A task at the topmost level is like a project, a small one or a big one. In the description you can enter the intention of the project and link it to your objectives. You should track not only your business projects, but also your personal projects, to achieve a work-life balance. To tidy up the cellar is really a project at home to be done in several steps.
A task may have the following attributes:
Next Action: a task may have dependencies, so often you couldn't complete it even if you had now time to do it. This is the difference to a Next Action, which a task without dependencies. In GTD, you ask "what's the next action to do", and if this action takes longer than a few minutes, you enter it in your system. And if you have sufficient time and are at the right location you can do this action whithout beforehand doing something else or waiting for another person doing something else.
The characteristics of a Next Action are:
Without the concept of the next action and it's context you will have a nice hierarchy of task's (call it todo-items), but what's to do next is burried in the hierarchy.