uuuh I think the internet can answer that better than I, haha. Once upon a time, the Christian church split into two thanks to a monk called Martin Luther who found that there were some things majorly wrong with his church at the time... Some agreed, some didn't, and Christianity split into Catholic and Protestant (or evangelist/lutheran as we know it here). The differences are, among other things, in their focus of beliefs, e.g. what parts of the bible they deem the most important. For catholics for example, Virgin Mary is an important saint, and protestants tend to focus on Jesus as the savior. But this is just very, veeeeery roughly explained. I'm sure you can find a better source than me lol.

As for being agnostic, and easy way of putting it is that I believe there is some higher power out there, but I haven't settled on it. So there's no faith per se I am following. I know here is something, or someone, but I am very uncertain and still searching for my answer, haha.

Hope that helps XD