Something I see a lot is writers using that and which interchangeably. Maybe they have used that already so they throw a which in, just for variety. Well, they're not always synonyms and the meaning of a sentence can change depending on which word is used. Here's a quick summary to clear up any confusion:
Use that to begin a defining clause:
Movie enthusiasts are going mad for films that feature strong female characters.
Here, the words that follow 'that' are defining the sentence object, films. The writer is using a defining clause to narrow down their definition of films. If you remove that feature strong female characters this sentence loses its intended meaning entirely: Movie enthusiasts are going mad for films.
An easy to way to determine if that should be used is to remove the information after it—does the intended meaning change? If it does, you should stick with 'that'.
Use which to begin a non-defining clause:
Movie enthusiasts are going mad for strong female characters in films, which means more films like this will be on their way.
In this case, if you remove everything after film, the meaning remains. The non-defining clause which means more films like this will be on their way, is extra information that isn't required to understand the entire meaning of the first statement.
Like with most 'rules', these are made to be broken. There will be times when which and that are both valid options in your sentence, in which case you just choose the one you like. But being aware of the impact 'which' and 'that' can have on meaning is important to avoid unintended miscommunications.
Kent Law's explanation, with exercises