Quizlet is an online flash-card tool that you can use to help your learners learn and practise vocabulary. And you can also use it to develop their pronunciation, grammar, speaking and writing skills.
The student version is free, and all your learners will need.
There is a pay-for teacher version with some very useful extra features. However, it's not an absolute must. I recommend that you start with the free version.
The Help pages are very good and show you how to use Quizlet in clear detail.