friend (male)

friend (female)

You're my best friend. (informal)

We are friends. 

partner (male)

partner (female)

acquaintance (male)

acquaintance (female)

She's an acquaintance. 

Where did you meet each other? (informal)

colleague (male)

colleague (female)

We are colleagues. 

classmate (male)

classmate (female)

neighbor (male)

neighbor (female)

stranger (male)

stranger (female)

I don't know him.