what I know is (since I was a salfi sunni myself) that Sunnis have four major groups , the differences between them are very few and not important , I would say it’s like the different between the rites in the church
for example what is the possition of your hand while you are praying , or how a person divorce his wife
however there is sect from sunnis developed the years called sufi which is very popular her in america , this type of islam embrace traditions (with small t) and ignore many of mohammed sayings and teachings , this type of islam have many false teachings according the the sunnies , such as asking prayers of the saints , or even having shrine and visit it , they celebrate the birth of mohameed and singing is popular in their mettings , This sect is influence by Christianity in general who by force left it . however , they themselves are divided to many schools of thoughts
Than the Salfi movement begin in the 18th and 19th century , which is calling muslims to go back to the quran and mohamed sayings , which is ‘’ True Islam’’ in their mind . This movement is the Fundamental islam , because they stick with the teachings of the early muslims generations .
this movement grow and wipe out the salfies whom their practices contradict what mohamed says .
however , suffi islam is growing in the west because of it’s ’ Peaceful Tone’ , which can’t fool any Salfi!
salfies believe you can’t ask any dead to pray for you , because that is shirk (worshiping person beside God)
Salfies are planning to re-establish the calaphat , which is kind like the papcy with more political rights . so the calipha is Mohammed on earth , because the word comes from claiphat rasul allah ‘’ the successor of the prophet of God’’
This is the Sunni islam
but Shia islam is different , they believe that mohamed have given the succession to ali and the 12 immam who success him , interestingly enough mohamed did say that , but sunnies intrepted differently
the hadith is : Surely Islam will always remain mighty till there are twelve caliphs in it. All of them will be of the Quraish
because sunnies used power to kill and destroy shiasim from very early time , for example mohamed grand son al hussain was killed by the calipha forces , and many of their immams were poisen and killed
they believe that the last imam (almahdy) went to a cave , and despire , and he will come again in the last days , and Jesus will pray behind him (it’s a sign that al mahdy is even mightier than Christ himself!)
and this is believed both by sunnits and shia , however sunnies believe the mahdy is not the shia one but another one
and sunnies belive this mahdy will defet the Christians and Jews and will protect islam .
Shia pray to saints , and visit their shrines , that’s why as a salfi we used to say they are infidels
Sunnis and shia hate each other , not even close to the level is protestant - catholic thing . I mean I have many protstant friends and we pray together , and sometimes we will use the internet to teach muslims about christ together
but salfies really hate shia , they believe they are jews in the cloth of islam !! and maybe they love christians more than shia , because shia believe that the friends of mohamed were not his friends but they betry him when he died and did not work by his saying to give the succession to Ali (the cousin of mohamed)
and they (shia ) still work with muta marrige (one day marrige ) while the sunnies don’t follow their prophet order to do such marrige because Oman the second Caliphate cancel it
Than you have newer groups , like the Quranic (Quran only ) which they believe all of us are muslims , and they destroy classical islam , and you have ahmadieas ,a sect made by some muslim in india who couldn’t defand his religion by the classical commtertor on Jesus , so he created a theory that Jesus was crucified( which is against the classic islam ) but didn’t die , but he went to india and died their , and he believe (the one who made this sect) that he is the Mahdi and christ but not Jesus , and this sect is very peaceful and don’t believe in jihad
so this is what I know about some differences , but you need to excuse me , for I am from Salfi sunni background and maybe there are more about shia and sufi from their side too