員警=警員=警察 They're all the same. Usually vocally we say "警察". In writing you can use either one. If you want to speak to a policeman, usually we said "警察先生" to be polite. --- This is according to the online dictionary by "Ministry of Education, R.O.C. " http://dict.revised.moe.edu.tw/cbdic/index.html

警察=警员=police ej:他是一个警察 （ he is a police. ）

员警 or 警员 seems popular in Hongkong, Macau and Taiwan, which appears in media quite often but is hardly used in Mainland. 警察 may be universal in the Chinese language area. 公安 may be a unique word in Mainland, which equals 警察. 差人、师兄（male）、师姐（female） are verbally used in Hongkong referring to police also, and 差人 is also used in Cantonese area but 师兄、师姐 are seldom used.

Basically, they both mean the same thing, but may be used in different situations. (A) If you want to say the "the member of police organization": The word "員警" is a formal word, and normally used in formal writings, such as government regulations, formal articles. News reports may like to use this word because is sounds formal. People use "警察" in daily conversation and any writings. (B) If you want to express "the police organization", just use "警察" in most situations. Hope it helps

没有“员警”这个词，有''警员"这个词，也有“警察”这个词，英语的police就是警察，警员不一定是警察，只是从事相关工作的人。