The short answer... Tasks are concrete activities that should be between 4 hours and 5 days long.

Why? Tasks or "Schedule Activities" make up the work packages of your WBS and are the lowest level of your schedule. As such tasks should have a defined outcome, an estimated cost, duration and a set of resources required. Each task in your project needs a who, does what to what level and when by.

Why should tasks be "...concrete activities..."?
Tasks need to be described by outcomes so you can judge when they are complete. Each task should have a measurable outcome and the expected level of the outcome. "Create steel framework to hold 2T of weight" is a concrete task. "Create accounting program" isn't, while "upload and verify all 5 million accounting transactions from legacy system" is. Measurable outcomes are required so you know when the task is finished.

Why should tasks be "...between 4 hours and 5 days long"?
At the minimum a task should take 4 hours. Anything less than that isn't a task, it's just an action. Actions like flipping a switch shouldn't be a task. Your schedule isn't there to program your team members like robots. As a PM it's not your job to micro manage your people, it's your job to help them achieve the outcomes of the tasks and thus the project.

At the maximum a task should take 5 days. Longer tasks should be broken down into smaller measurable tasks that are achievable in less than 5 days. Having a maximum is just as important as not micro managing. Any tasks longer than a working week often gets put off until the next week, or the week after. Tasks and their completion on schedule are an important indicator of the health of your project, if your tasks are longer than a week it'll be too late when you find out you are weeks behind in schedule.

To overcome the problem of long tasks, people often use "Percent Complete" to monitor the completeness of a task. The result is often that tasks are 90% done for 200% of the scheduled time. When using this i have often noticed that the last 10% often takes 190% of the scheduled time. Percent Complete isn't a waste of time but should only be used as an indicator for reporting on WBS work packages. It should not be used to monitor tasks. Tasks are either done, or not done.

How do tasks relate to the WBS?
Tasks or using PMBOK terminology: "Schedule Activities" are not part of the Work Breakdown Structure. The WBS should be devolved to distinct Work Packages which each have a distinct deliverable. The WBS stops at the Work Package level or deliverables. Typically these deliverables take several activities to complete, so in the schedule the Work Packages are devolved into discrete activities (or tasks) with outcomes, estimated duration, cost and resources with relationships to other activities in the Work Package. ie: Who does what and to what level and by when.