演习，is used in a military context, like an army drill; 练习, is pratice that we do to make progress in a certain subject. There are some other similar words like 演练、排练, which mean rehersal.

I think, and please check with a native speaker, but my understanding is 演习 is to do with official practice, things like army drills, fire drills etc. For example 军事演习 is an army manoeuvre/war game.

My English isn't good . Maybe some translations is wrong .Hope i can help you . 练习 /liànxí/ 1. 动词 V. 反复操作，求得纯熟。 「练习技术」 Repeated operation to obtain skill. "Practice Technology" eg:让我练习下我今天学到的知识。（Let me PRACTICE what I learned today.） 2. 名词 n. 为巩固所学知识而设置的作业等。 「基础 练习」 homework set up to consolidate what you have learned. "Basic Practice" eg:这是我们今天的课后练习。（This is our post-class PRACTICE today.） 演习 /yǎnxí/ 动词 V. 按照预想的方案进行实地练习（多用于军事方面）。 「演习海上救护」 Practice in the field according to the envisaged plan (used mostly for military purposes). "Drilling at Sea" eg:这将是2015年以来中国在这一水域进行的首次海军军事演习. (This will be China’s first naval military EXERCISE in this waters since 2015.)

演习，is used in a military context, like an army drill; 练习, is pratice that we do to make progress in a certain subject.