Use "which" with either the defining clause or the descriptive clause.
Use "that" only with a defining clause.
Examples:

The movie that I saw last night was enjoyable.

-- "That I saw last night" is a defining clause; "which I saw last night" could also be used here.

The movie, which I saw last night, was enjoyable.

-- "Which I saw last night" is a descriptive clause.

She ate the apple which was in the bowl.

-- "Which was in the bowl" is a defining clause; it defines which specific apple.

She ate the apple, which was in the bowl.

-- "Which was in the bowl" is a descriptive clause; it describes the location of the apple but doesn't specify which apple. Note the use of the comma.