两种从句的区别主要在以下三方面:
▶从词类上区别
同位语从句前面的名词只能是idea, fact, news, hope, belief, suggestion, proposal, word, thought, doubt, truth, possibility, promise, order等有一定内涵的名词，而定语从句的先行词可以是名词、代词，主句的一部分或是整个主句，如: The possibility that the majority of the labor force will work at home is often discussed.人们经常讨论大多数劳动力将会在家里工作的可能性。(同位语从句)
Our team has won the game, which made us very happy. 我们的队赢了,这让我们很高兴。(定语从句)
▶从性质上区别
定语从句是从句对其先行词的修饰或限制,属于形容词性从句的范畴;而同位语从句是从句对前面抽象名词的进一步的说明和解释,属于名词性从句的范畴，如: The news that our team has won the game was true. 我们队赢了那场比赛的消息是真的。(同位语从句,补充说明news到底是一个什么消息。)
The news that he told me yesterday was true. 昨天他告诉我的那个消息是真的。(定语从句,news在从句中作told的宾语。)
▶从引导词及其在句子中的成分上区别
有些引导词如how, whether, what可以引导同位语从句,但不能引导定语从句，如:
That question whether we need it has not been considered. 我们是否需要它这个问题还没有考虑。(同位语从句)
引导词that引导定语从句时,在从句中一般做主语或宾语(指物时还可以用which代替),并且作宾语时常常省略，that在同位语从句中仅起连接作用,不充当任何成分,并且不能省略,也不能用which来代替，如:
The order that we should send a few people to help the other groups was received yesterday. 我们应派几个人去帮别的几个小组的命令昨天收到了。(同位语从句,是对order的具体解释,that虽不作成分,但不能省略。)
The order that we received yesterday was that we should send a few people to help the other groups. 我们昨天收到的命令是我们应该派几个人去帮助别的几个小组。(定语从句,是名词order的修饰语,that在从句中作received的宾语,可以省略。)