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that
vs which
vs who

"Who" (or whom) is a pronoun, and is used as the subject or object of a verb to show which person you are referring to, or to add information about a person just mentioned. It is used for people, not things.

"Which" is a pronoun, and is used as the subject or object of a verb to show what thing or things you are referring to, or to add information about the thing just mentioned. It is used for things, not people.

"That" is a pronoun, and is used as the subject or object of a verb to show which person or thing you are referring to, or to add information about a person or thing just mentioned. It can be used for people and things. It can sometimes be omitted.

Example:

The girl who was hungry.
The boy whom I talked to.

The dog that wagged its tail.
The software ( that) I wrote.
The company, which / that hired me.