杯 if we use alone,it is a measure word(量词）. eg 一杯咖啡 （a cup of coffee）一杯水（a cup of water) 杯子 it means cup or glass eg 这是我的杯子。 这个杯子很漂亮。

Actually, 杯 is a nouns, is a letter, & we do not say 杯 alone, so we would say 茶杯, means a cup to store tea. we would say 酒杯，means a cup to store wine. But, 杯子 is a phrase, we can use it directly, like, pls pass the 杯子 to me.

杯 is 杯子 without 子