Basically, a web site is a set of documents (called 'web pages') that you can view via a computer network called the Internet.
Web pages are in a format called HTML (HyperText Markup Language, if you really want to know), and the program that you usually use for viewing these documents is called a web browser.
If you're viewing this web page in its original location on the Internet, then you at least know the basics of how to view such documents, even if you can't explain exactly what a web site or a web page is.
There are two things that make web pages different from other documents that you view on your computer (say, a word processing document or a spreadsheet):
In principle, creating a web page differs little from creating any other type of document on your computer, such as a word processing document. In reality, just like you need to know how to operate your word processing program in order to write a letter with it, you need to know something about web pages and the programs used to create them in order to create them yourself.
So, if you want to create a web site for your religious organization, you'll be creating a set of documents about your organization, and these documents will probably contain links between each other.
Speaking of links, follow this one to the next topic: Web Site Features