1. A method comprising: at a multi-tenant work distribution system that provides a work distribution service for a plurality of external systems, the work distribution system including a multi-tenant account module, a RESTful worker Application Programming Interface (API) module, a worker state module, a RESTful work item API module, a work collections module, and a work distribution engine: responsive to receipt of a RESTful work item API call request by the work item API module, the work distribution engine assigning a priority to a first work item of a first external system, the first work item being received via the RESTful work item API call request, the work distribution engine assigning the priority based on work item attributes of the first work item and a workflow instruction corresponding to workflow information specified by the first work item, the workflow instruction being provided by the first external system via a RESTful workflow API module of the work distribution engine; the work distribution engine assigning a worker to the first work item based on: the priority of the first work item, the workflow information, and worker state managed by the first external system via the RESTful Worker API module, the worker state including worker attributes, wherein the first work item is generated by the first external system, and the workflow instruction is managed by the first external system, wherein the work item attributes of the first work item are specified by the first external system via the work item API module, wherein the worker attributes are specified by the first external system via the worker API module, wherein the first external system defines the workflow instruction in accordance with a syntax defined by the work distribution system, wherein the worker state indicates at least one worker attribute and at least one worker status, wherein each of the plurality of external systems is associated with at least one work distribution account of the multi-tenant account module, wherein the first work item corresponds to a first work distribution account of the first external system, wherein each work distribution account of the multi-tenant account module is associated with at least one set of work distribution primitives, wherein work distribution primitives include workers, work items, work distribution workflows and work collections, and wherein the first external system manages the work distribution primitives associated with the first work distribution account by using at least one of the worker API module, the work item API module, the workflow API module, and a RESTful work collection API of the work collections module.