晚上指6pm~6am，尤其指6pm~12pm 夜里指8pm~4am，尤其指深更半夜 但是这个区别不是绝对的，大部分情况下两个词可以换着用。

The two words mean the same but "晚上” are used more often in speaking.

like zuotengdazuo said, 晚上 is from afternoon to evening. For example, 你晚上吃什么？ 你晚上有什么安排？We use 晚上 in our daily expressions. 夜里usually means at night.For example, 夜里很安静。我夜里睡不着，失眠了。

Actually the difference between 晚上 and 夜里, mostly exist in the phrase. For example, good evening we say 晚上好 not 夜里好. And 晚上 is more frequently.

ummm..... and what's the difference between night and evening? e_e