What is the difference between Rhino and Spidermonkey JavaScript engines?

Question

For the first time, I began learning Javascript, however on the start I stuck up with two possible options: Rhino and Spidermonkey. Could you please, tell me what is one, and what is another, so I can easily choose for myself the best option that suits my needs. If it makes easier for you, you can list advantages and disadvantages of both Javascript versions.