this evening是指今天晚上的早些时候，tonight是指今天的整个晚上包括深夜。前者是今天傍晚，后者是今晚。前者在时间上要早一些。
this evening例句
1、Let's go out this evening.
咱们今天晚上出去吧。
2、You have a point ─ it would be better to wait till this evening.
你说的有道理，还是等到今天晚上比较好。
3、It's nice to see some new faces here this evening.
今晚在这儿见到一些新面孔真是太好了。
tonight例句
1、I thought we could go out tonight.
我本想我们今晚可以出去。
2、I'd like to book a table for two for 8 o'clock tonight.
我想订一张今晚8点钟的二人餐桌。
3、I'd like to take you out to dinner tonight.