Nobody really knows. There aren't many speakers compared to the "big" languages, but there are enough that you can use the language in lots of ways. For a few concrete examples, check out the page on Fields of Use.
It depends how you look at it... Many of the root words come from European languages, but the grammar of Esperanto has many traits that are atypical for European languages, which make it somewhat similar to perhaps Turkish, Swahili or even Chinese.
Compared to other languages, certainly. But as with learning any language, some find it easier than others. You're certainly at an advantage if you already speak another foreign language.
In the beginning, the language was called Lingvo Internacia - the international language. When Zamenhof introduced the language, he used the name Dr. Esperanto (it means 'doctor who hopes'). Some people started to call it "the language of Dr. Esperanto", which was eventually shortened to just "Esperanto", which is the most common name today.
Yes, a few in fact. The green star has traditionally been used as a symbol of Esperanto, and it's a part of the Esperanto flag. The color green is a sign of hope, and the five points of the star symbolise the five continents.