While those answers are correct, I thought about this for awhile, and I think that for every usage of "Likewise" you can replace it roughly with "in the same way". Ditto is very colloquial and just means you completely agree with the previous comment in the same context. I would actually avoid using Ditto at all costs if you are learning English.

Another use for likewise: Person 1 : "nice to meet you" Person 2: "likewise"

They can be used in a similar sense in an informal conversation. For example if someone says to you: "I like Italian food". You could reply "Ditto" or "Likewise" (if you also like Italian food that is). However 'likewise' is also used in sentences in a way that 'ditto' cannot be used: "I found French hard to learn. Likewise, I am struggling with Italian." Hope that makes sense.

*What's the difference beteween these words?