For your first question: dojo is javascript-based platform that provide you with a basic javascript library and a bunch of basic widgets (UI controls like button, dialogue, layouts,...), and some extra things. However, you don't really have to use dojo all the time: you still can use dojo to manipulate a html form button; it's just dojo button comes with extra functionalities and might save you some time.
For the second question, I agree with iH8 that dojo website is the best place to start. There are three different ways dojo websites can help you:
Look at the tutorial: Basic steps on how to set it up and use provided functionalities as-is
Look at thetoolkit api: This provides a very detailed view of dojo javascript object (See what extra things you can do with dojo objects)
- Look at the nightlytest: I found this very helpful in term of showing me what can be done outside of the tutorial (i.e. how to use things you found in the api)
Other than these, you can look at existing implementation to learn about the toolkit.
Basically, this is how I am learning Dojo. Without more-specific questions, it's hard to tell what is confusing about the tutorial. I would recommend you give it a try and post a question: everyone here will be willing to help you.