What is the difference between watching pornography and watching indecent films, and is watching TV series that display nudity equivalent to watching indecent films in sin?

Your distinction between "pornographic films" and "lewd films" is not clear, as both terms refer to the same thing, and watching them is haram (forbidden), and perhaps considered one of the major sins. The sin increases according to the prohibited content being watched. So, the sin of watching full indecency is greater than the sin of watching kissing or embracing.