Flashcards have been used for a long time by students that want to broaden their vocabulary, whether for learning a second language or increasing one's vocabulary of your primary language. Before computers, students often used index cards and wrote a word on one side, and the meaning of the word, or the equivalent word in another language on the other side. Now there are all sorts of flashcard websites and flashcard software programs available that basically do the same thing electronically.
But regardless of the medium you use, there are some ways to use flashcards that are better than others. I'm going to recommend one way that I find very helpful. Instead of just randomly selecting 20 or 50 or 100 words, and trying to memorize them via flashcards, select vocabulary words that are in context. What this means is that it's better to create flashcards based on a reading passage or book or essay you've read, and then selecting words from what you've read and creating flashcards for them. That way, you have a good chance of associating the word with a story or book you've just read, so the word will be more meaningful and you'll learn it faster.