What are the Differences Between Sunnis and Shias?

Twelver Shiism, otherwise known as "Imami Shiism," is the second major branch of Islam. Imami/Twelver Shiism is by far the dominant Shia sect- so it is proper to refer to Twelver Shiism/Imamism as just Shiism. For the most part, Shiism and Sunnism are largely the same in terms of basic beliefs- both believe in One God, who sent the Quran with the Prophet Muhammad who is the final messenger, and both believe in the obligations of prayer, Hajj pilgrimage, zakat (charity), and all other obligations laid out in the Quran and the agreed on Sunnah (practices, words, and actions) of the Prophet (s).

There are, however, some important points of difference which have resulted in differences over the details of practice and beliefs between Sunnis and Shias.

Shias and Sunnis differ on the concept of Khilafa (Caliphate).

At first glance, the split on successorship over whether Imam Ali or Abu Bakr was rightful to rule after the Prophet seems like a small political difference, but this is based on a more fundamental theological difference.

Shia and Sunnis differ on where they get Ahadith from.

Ahadith are the recorded sayings and actions of the Prophet which were transmitted to us through various narrators. The split between Sunnis and Shias on this issue is on two points.

This is not to say that Shias view all Sunni hadiths as automatically false, but a Shia would take a more skeptical eye towards the narrations recorded in Sunni books.  But for the most part, a majority of the Sunni Hadith corpus is largely unproblematic for Shias, and much of the contents are found in similar hadiths in Shia books of Hadith.

Shias and Sunnis differ on Theology regarding God's attributes and Essence.

Shias fairly unanimously believe in Classical Theism, Divine Simplicity, the transcendence of God beyond space and time, the fundamental Goodness and Justice of God, secondary causation and concurrentism, and the primacy of reason over the text. (Usul al-A'imma; Ja'far Subhani, Doctrines of Shiite Islam). Sunnis, however, are split into three major schools of theology, each having its own set of beliefs which may or may not align with Shia theology: Asharis, Maturidis, and Atharis. Some basic differences between Asharis and Maturidis can be found in "What Are the Differences Between Asharis and Maturidis," and "12 Differences Between Asharis and Maturidis." Differences between Asharis and Atharis can be found in the video "Differences Between the Ashari and Athari Schools."