"职员" is more commonly used when it is related to job/occupation. If you are answering question like "what's your job?", the word "职员" would be more natural to use. "员工" is more like a wider name for all workers inside a company, "I work here" -- "我是这里的员工。" "All members in this company are very excellent." -- "这家公司的员工都非常优秀。" Base on my personal experience, "职员" is more formal and rarely used in daily talks, but "员工" can be used in any situation. But as Iris said, their difference is very small.

To me, 员工 and 职员 几乎没什么区别。 I agree with 妙脆角 that 公司职员 is more senior than 公司员工. (However,to be honest, I didn't notice this until I did some research online myself. And I am a native Chinese speaker.) If you google or baidu(it's the chinese version google) 超市职员, 商场职员,学校职员 you will get a lot of results. Some of them are even from official government documents. For example, http://news.sina.com.cn/culture/2000-1-10/50913.html https://wenku.baidu.com/view/12d3f4d1360cba1aa811daa2.html If I were you, I probably won't bother to remember the differnce between these two words. It's really trivia.

Since Rui has already given the best answer, I just want to make a supplement. The two words are often used interchangeablely. 公司职员 is more senior than 公司员工. Generally speaking, 公司职员 do contribution to the administration or the business of the company, while 公司员工 can almost be used to refer to everyone in the company except the boss. For example, security guards and cleaners of the company will say they are 公司员工 instead of 公司职员。 Here list some other cases that 员工＆职员 can not be exchanged： 食堂员工√ 食堂职员x 超市员工√ 超市职员x 商场员工√ 商场职员x 学校员工/职工√ 学校职员x

他们几乎没什么区别。