The Shia are an Islamic Sect

Sharh Muwaqif by Imam of Ahlul Sunnah Meer Seyyed Shareeh Jurbani:

Amongst the biggest Islamic Sects the other is Shia, Shia are those that follow Ali and believe after Rasul he was the rightful Imam and Khalifa whether that be spiritual or physical, and they believe that Imamate and Khlifath remains in the family od Ali page 752 (Lucknow print)

Abu Saadath Mubarak Ibn Aseer Jazari in Jami al Usul Min ahadith ur Rasul Volume 12 oage 220-221 (gytpt Edition)

We are talking about famous Sects, that Muslims affiluiate themselves to these are as follows, Shaafir, Hanafi, Malik, Ahmad and Imaiya.