Vocabulary is all about knowing more words and their different usage. Research has shown that IQ test and vocabulary test resemble similar results. That apart words represents your ideas. If you don't use right words to describe your ideas, you really lose effectiveness in speech and writing.
Here are the few resources that can help you build and reinforce your vocabulary.
Vocabulary.com
This website is the really the only thing you need to improve your vocabulary. All you have to do is create an account with them. If you spend 20 to 30 minutes a day with this site, you would see that your word treasury is growing. The best part about this site is its ability to remember. If you give wrong answer to any question, that word is stored in the memory and site would put that word before you every now and then until that word is all yours.
That apart site has number of already made list that you can use to learn words. Also you can create your own list of words as per your need. Site has a very well statistical representation of your progress. Also, site serves as one of the finest dictionary. Meanings are explained in an awesome way so that you can remember it. Here is an example:
Wordsmith.org
If you are looking for "a word a day" service, this website is for you. Wordsmith is founded by Anu Garg who is one of the well known personalities in linguistic arena. Website emails you a word a day. Each week is dedicated to some specific theme of words. Anu Garg has also authored a books like A word a day and Another word a day which is collection of few of the most intriguing words in English language.
Merriam-Webster.com
Merria-Webster is one of the most popular dictionary resource in offline world. However, if you love words or want to learn words because you need to, its wordlist can be the great way to learn. All the words are described along with images that helps you enjoy the learning process. Site also have many games like Spell it and visual vocabulary building test.