To use the vocabulary lists (and create your own), you need to register (free). You have to be at least 13 years old to register. Once you register, you can keep track of the words you're learning. And the system keeps track of what you've already done so it can help you practice and improve.
There are lots of lists of words that are already put together, at different levels of difficulty. For example, if you like The Hunger Games but you stumble over some of the words, you might decide that it would be helpful to use "The Hunger Games" Vocabulary from Chapter 1.
You can look over the list of words ("canvas," "cocoon," "insist," "scrawny," etc.). Or you can look at a list with definitions. Or a list with definitions and notes, including the sentences from the book where the words are used. Want more? Click and you can read notes about any word. For example, for "venture":
As either a noun or verb, venture implies risk. Your family won't like it if you leave school to go on an artistic venture. Those who chose to venture off school grounds were never seen again.
You've probably noticed that venture is a shortened form of adventure. This happened sometime between 1100 AD and 1400 AD during the time that Middle English was spoken. While the two words are similar in meaning, when you subtract the "ad," you lose a teaspoon or two of fun, and add a heaping tablespoon of risk.The same page offers more detailed definitions. On the right side, you can click to see the word used in different sentences from the news, sports, business, science, and so on.
You can try out your vocabulary by clicking on "Learn this List." You get a series of multiple-choice questions about the words in the list. If you guess right, you get a cheery message ("Nice work!"). If you don't, you get another chance, and another. You can also ask for hints—either having the program eliminate two wrong answers or give you a sample sentence. When you don't get a word on the first try, the system remembers it, so you'll get a review later.
After 10 questions, you get a score. You see your best streak (e.g., 6 in a row), and you get a bonus if you got all 10 right. Then you can keep going to the next round. Getting the scores so often makes it into a game. It's fun to try to get a long streak going or to get a bonus for a perfect round.
After you feel pretty good about the vocabulary from chapter 1, then you can move on to the list for chapter 2, and then chapter 3.
There are lists for famous speeches (e.g., Franklin Roosevelt's "Four Freedoms" speech, a speech by Nelson Mandela) and lists for literature (e.g., Walden, The Great Gatsby).
And there are lists designed to help students prepare for standardized tests, like the SAT.
Vocabulary.com is produced by Thinkmap. Thinkmap has other products that require a subscription, but Vocabulary.com is free. Here is Thinkmap's press release from when it launched Vocabulary.com (April 2011).
In 2012, Vocabulary.com was an Official Honoree in the Education category of the Webby Awards.
The more I use this site, the more I like it.