Reddit mentions of Sufism: A Beginner's Guide

Hello person! I hope I can help you a bit here.
Just a warning, a lot of the answers you get will be tinted based on our own beliefs.

Overall, most Muslims are Sunni. Hanafi and Hanbali are actually 2 of the 4 main legal schools (fiqh) within Sunni Islam, along with Shafi and Maliki. I wouldn't compare these to Protestant sects because (AFAIK) the scholars from these 4 different schools of fiqh generally accept each other's validity. Although, I don't know if that's always been the case historically.

Sufism is difficult to define. I've heard different roots of the English word "sufi" but the one I hear the most is tasawwuf which is the science of purifying one's heart. Sufism isn't really a sect, like /u/Azeem259 says, Sunnis and Shias can be Sufis. It has to do with pursuing spiritual excellence, becoming close to God.

It's so diverse though and means a lot of different things to different people. There are traditional Sufi schools with teachers who follows chains of scholarship and worship that go back to the Prophet SAW. Some local sufi practices in some places are really iffy for other Muslims, like veneration of saints and their tombs. But again, that's some groups, that's not the heart of sufism. Sorry my thoughts about this are so scattered. There's a cool intro by William Chittick called Sufism: A Beginner's Guide.

Salafism (or Wahhabism if you want) is a school of thought (or rather, methodology) in Sunni Islam in which the practitioner strives to be as close to the Prophet SAW and his Companions in every aspect of his or her life. But even in this group, there's a lot of diversity. I don't want to talk too much about it, because I know very little, but Shaykh Yasir Qadhi wrote a paper on it.

I think it depends highly from which perspective you are looking into it:

If you are a conservative muslim and you are searching legitimization/answers to questions and how sufism is related to traditional understandings of Islam, do not look further than: "Realities of Sufism", by Sunnipubs.

If you are not like that, but you are in search of a good introduction, not overwhelming, not too technical, then I'd say go with William C. Chitticks Sufism - A Beginner's Guide.