关系代词有who, whom, whose, that和which, 是用来引起定语从句的。who, whom代表人,在从句中作主语时用who, 作宾语时用whom。这些关系代词,一方面引起定语从句,修饰前面的那个名词,一方面在从句中担任一个成分。(who作主语,whom作宾语,whose作定语。)
关系代词是用在定语从句里的,比如I like the book which I bought.里的which;
连接代词是用在名词性从句里的,比如I don't believe what is report on TV.里的what。
它们本质其实都差不多,都是充当从句里一个名词性成分。前一句定语从句里,which充当的是从句里宾语成分,后一句名词性从句,what充当的是从句的主语成分。