Strange is an adjective meaning odd, not normal, weird, etc.

Stranger can be an adjective meaning the same as above, but even more so. People may use "stranger" or "more strange."

Stranger can also be a noun meaning someone you do not know. This definition is much more commonly used than the one above.

It can be.

For example:

This soup taste strange, but the soup I ate yesterday day tasted even stranger. - in this case "strange" is an adjective, and "stranger" is the comparative adjective. 

However, "stranger" can also be a noun, as in someone you don't know. 

For example:

Person A: Do you know that guy?

Person B: No, he was a stranger asking for directions.

strange is an adjective describing something/somebody.

a stranger is a noun when describing someone you do not know

"the word stranger is an intensifier of strange."

the word stranger is used as an intensifier, sometimes, but usually as a comparison.