When I was a child, my mother taught me not to get into a car with a stranger.
When my car broke down, a kind stranger stopped to help me.
A foreigner is someone who comes from another country:
This town is a popular tourist destination, so there are always a lot of foreigners around.
The new law makes it easier for foreigners to get documents to work legally.
A lot of foreigners have been moving into this neighborhood - I've met people from seven different countries in my apartment building alone!
The word alien is a legal term for foreigner. It is usually used to describe "illegal aliens" (people who are in the country illegally) or "resident aliens" (people who are living in the country legally). Again, this is formal/legal language and in everyday speaking it's best to use foreigner.
Alien also refers to creatures from other planets; it is another word for extraterrestrial.