work contract 其实就是 contract for service :Definition
Contract for a job undertaken by a self employed person.While he or she may hire others to carry out (or help) in the work,he or she is solely responsible for the satisfactory completion of the job.
service contract 是指 contract of service:Definition
Oral or written,express or implied,agreement specifying terms and conditions under which a person consents to perform certain duties as directed and controlled by an employer in return for an agreed upon wage or salary.
建议您通过谷歌使用 difference between contract of service and contract for service 搜索,就可以得到许多资料,够您参考的了.
【英语牛人团】非常感谢大神相救呀~~~~可以用LAW的角度分析下么我手头也没有资料。但 law 是各国都略有不同，还是要根据你的需求去搜索。