What types of essays are there, and when do you use each one?

It's an assignment.

http://www.google.com/#q=types+of+essays

Thanks, and when you have to define just one word such as anything, what type would it be?

Which one do you think it is?

http://www.time4writing.com/writing-resources/types-of-essays/

I assume that is persuasive essay, but even though if every fact is based all on science fiction information-they still act as actual facts?

Why would you try to persuade someone about a definition?

I think it would be an expository essay.

"3. Expository Essays: Just the Facts
The expository essay is an informative piece of writing that presents a balanced analysis of a topic. In an expository essay, the writer explains or defines a topic, using facts, statistics, and examples. Expository writing encompasses a wide range of essay variations, such as the comparison and contrast essay, the cause and effect essay, and the “how to” or process essay. Because expository essays are based on facts and not personal feelings, writers don’t reveal their emotions or write in the first person."

--http://www.time4writing.com/writing-resources/types-of-essays/

I kinda get it, so it'll be an expository essay to write about, what is a monster, and define it what I think is a monster?, sorry that's what I'm still struggling.

Yes. That is definitely an expository essay.

Perfect, and as far as I know I have to write a thesis, do I have to write for every type of essay? as well for an expository essay?

A thesis statement is necessary for all types of essays.

This site has a lot of valuable information about thesis statements.

http://leo.stcloudstate.edu/acadwrite/thesistatement.html

Alright! I really appreciate all your help. As my whole essay is based mostly on thesis statement. In my case, I have to define what a monster is, so in order to come up with a thesis statement I have to say something I basically think about it, is it good if I said something as

"Every monsters is cruel and evil, and more importantly it the reflection of humans" for my thesis statement.