Task is a generic construct while service task is dedicated to operate on some sort of service (for instance a web service or other implementations). Service task has some requirements such as specifying implementation and operation reference (meaning to define interface and its operation and corresponding messages) and task does not have those. In jBPM 5 task can be used to plug in your custom nodes (domain specific service) that extend the default set of available nodes and gives you freedom of designing the node as you like.
HTH