Passionate means you love doing something, and you spend a lot of effort and energy on it. Interested is very similar meaning, but less strong than passionate. It can also mean you are still thinking about doing something but haven't started yet, for example 'I am interested in taking English classes', could mean this person has not taken any English classes yet, but would like to try it out.

It depends somewhat on the context. But for example "I am interested in learning Spanish" pretty much means "I want to learn Spanish." "I am passionate about learning Spanish" means that I devote a lot of time, energy, and thought to learning Spanish. Do you have an example that you are interested (want) to know more about?

In this context, "passionate" represents a stronger degree of involvement than "interest." Examples: "I am interested in the Romantic composers, but I am only passionate about Beethoven." "I find Henning Mankell's detective novels interesting, but I'm not really passionate about them." "I told Weight Watchers, 'You can take away my potato chips, you can take away my bacon, but don't mess with my doughnuts--I'm passionate about them!'"