Scarleteen

What's the difference between cumming and squirting?

When people talk about "cumming," sometimes that refers to having an orgasm, and sometimes it refers to ejaculation. "Squirting" is a colloquial term for ejaculation when done by someone with a vulva (also called "female ejaculation" although not everyone with a vulva is female). We have some information about that here: Squirt: On Ejaculation.
So, the difference is going to depend on what someone means by "cumming;" ejaculation can happen with orgasm, but doesn't always, so it could be that someone's referring to two different things that could happen around the same time or that they're referring to the same thing.

Also, during sexual arousal my vulva produces this creamy white substance. What is this? Is that not 'cum'?

That sounds like vaginal discharge, which can be more noticeable during sex, or if you're paying a lot of attention to your genitals. You can read more about discharge here: Honorably Discharged: A Guide to Vaginal Secretions.