Scheduled Tasks are regularly timed tasks that can run every few seconds, every day, every week, etc. See Admin-->Manager Scheduled Tasks for the administration of them.
A task is just a Java class that is registered in OpenMRS by being in the scheduled_task_config table. Typically Modules provide these tasks.
You can edit the properties of a task by clicking through to it.
Prior to v1.7 of OpenMRS you had to set a username/password used to run tasks in your openmrs-runtime.properties file. This lets the tasks authenticate to openmrs to do their things. (After 1.7 you can delete this entry because tasks run as the "root" user by default)
There are a number of default tasks provided: