Simple questions about HTML/CSS etc

[New User]

Could somebody please explain to me what the following topics are?

HTML
CSS
PHP
Java(script)

I know it has something to do with coding websites etc, but what are the differences between them?
If they are just different languages to code websites, what's the pro & con with them?
Why should I learn one of them and not the other?
I've seen many people who learns several languages, why would they need that? Isn't it enough with one?

 

[codemonkey]

They are completely different things, chances are your website will be ugly without at least HTML & CSS.

http://css-tricks.com/inapproprite-uses/

This covers it pretty well, however I'll add PHP is for connecting to databases and doing stuff "behind the scenes."

 

[JerseyFoo]

It's JavaScript. Java is an overrated language used primarily for 'research'.