How are postage prices calculated by USPS?

Do they go by weight? I also know that it depends on what kind of item you’re shipping sometimes. I’ve been to the post office after looking up a price on usps.com, and they told me there that the price was significantly more than the website had said it would be, so what’s up with that? I know I wasn’t looking at the online price, because it said “Post Office Price” above the price.
It’s less likely that I was using the site wrong than it is that their website just needs to be updated.
Either way, their system is confusing me. Whatever I hear seems to always be contradicted by some other thing I hear, and I don’t know where to go to start learning, other than here. I want to know how not to pay too much before I ever try it, not after I’ve already wasted money, so trial and error isn’t an option for me.