IN THE NAME OF ALLAH THE COMPASSIONATE THE MERCIFUL.

Dear noble brother

We have received your questions by information net (internet) and it contains the following questions:
(1) What is the difference between the Sunnism and Shiism ?
(2) Is true that Sunnis are (Kufar)disbelievers according to Shia's believe?
(3) What is my duty in my way to know much about Shiism ?
(4) Why Shia is better than Sunni?
These are your questions, but let us deal with the answers in summarized :
To answer on the first question, we say: Both Sunnis and Shias has agreed on three principles which are the basis of the believe and accession to the pound of Islam. Those three principles are:
(1)(al- Tawheed) Unity of God
(2)(al-Nubuwah) Prophethood.
(3) al-Ma'ad) Resurrection.
Verily the difference is in other issue, In fact after the Prophet ( P.B.U.H) the muslims have agreed on the source of Caliphate and Imamate, but the disagreement was focused on the position ( Caliphate), is this special position is a Divine's position that it needs to be appointed by the Almighty Him self or it can be appointed by human being through nation's election ?
From this point Shia believe that the Prophet (P.B.U.H.) appointed Ali bun Abi Talib ( peace be upon him) with the command from the Almighty allah to be a leader of this nation after him, and this happened in a great gathered during his coming back from the Farewell pilgrimage in the tenth year in a place called Ghadeer Khum, when command came from Almighty Allah saying : O, apostle proclaim the message which hath been sent to thee from the Lord. If thou didst not, thou wouldst not have fulfilled, and proclaim his mission, and Allah will defend thee from men ( who mean mischief) For Allah guideth not those reject faith. ( al-Maaidah - verse 67 ).
Indeed a huge group of Sunni's commentators or Quranic translators has mentioned about the revelation of the verse together with the incident.
As for the Sunnis they do believe that after the Prophet (P.B.U.H.) the caliphate position depend on the nation it self not on the Divine as Shia believes. So this is the essence of the disagreement between the both sects, so in this case he who believe that the Caliphate of Ali bun Abi Talib was appointed by the Prophet (P.B.U.H.) from Almighty Allah then the fellow is Shia, and he who believe the opposite of this is not Shia.
THE SECOND ANSWER:
Verily both Sunnis and Shias are Muslims because the limitation of the believe is consisting in those three principles and all of them are common between both sects, so he who said Shias are saying that Sunnis are (kufar)disbelievers then he had fabricated on them.
THIRD ANSWER:
You have already knew the essence difference between the both sects, so you and all those who want to know much about Shiism have to refer or review to these books which was written by famous scholars of Shia, and we will mention these particular books as follows :
(1) (Aslu-Shia wa usooliha) - Genesis of Shiism and its principles - written by Shaikh Muhammed Husain Kashif al-Ghita'a.
(2) ( Al-Muraaja'at) - Refferings .written by Sayyid Sharafudeen al-Aamuli.
(3) Al-Aqeedah al-Islamiya) The Islamic Believe - written by Shaikh Jafar al-Subhani.
(4) Thummah Ihtadaitu) Then I was guided - written by Dr. Muhammed Tijani al-Samawi, and other examined books.
FOURTH ANSWER:
Verily Shias use to take their principles, its positive laws, principal features of religion and laws from the Holy Quran and the Sunna of the holy Prophet, means his practices as a first source.
And take from the Ahlulbait Imams (peace be upon them) the clean progeny, those are equal to the Holy Quran as second source, as was related from the Holy Prophet (P.B.U.H.) and is widely transmitted saying :Iam leaving you with two weights, one of them is the Holy Quran and the other one is my progeny.
But as for why they are called Shia, in fact this refer to the Holy Prophet ( P.B.U.H.)where the commentators or Quranic translators have mentioned that after this blessed verse was revealed saying : Those who have faith and do righteous deeds, they are the best of the Creatures. (al-Bayyinah verse 7 ) Then Prophet (P.B.U.H.) said to Ali bun Abi Talib ( peace be upon him ) that is you and your followers are pleaseds and pleasants ( please refer to Shaikh Suyuti's book called al-Dur al-Man thur vol. 2 page 589. or al-Sawaiq al-Muhriqah page 161.
Before to end we are supplicating to Almighty Allah to grant you success and prosperity.