the basics-
Sunni- believes that Abu bakr was the successor of prophet Muhammed.
Shi'ia- believes that Ali ibn abi Talib was the successor of prophet Muhammed.
between the two sects, there are differences in religious practice, traditions, and customs, often related to jurisprudence.
The sunni sect is further subdivided mainly into Shafi'i, Salafi and Hanafi'i schools of thought. This division is important to understanding islam in kerala.
Islam in Kerala-
Almost all muslims in kerala are part of the sunni sect. shia islam is non-existent in Kerala. most malayali muslims' knowledge on shia islam is zero.
What does orgs like Samasta, jamaat islami do?
this is where the sunni schools of thought i said earlier comes into play.
Samasta(Shafi'i)-
They are the principle Shafi'i scholarly body in kerala.they have the most sizeable followers in the state.they also control most of the mosques and madrasas. it was formed in 1920s as a response to the growing radical salafi movement in kerala.
there are two factions within the organisation based on political differences- samasta EK and samasta AP.
EK faction supports UDF. one could even argue that IUML is the political wing of EK sunnis, but party leaders often despise that statement.
AP faction mostly supports LDF.they are headed by a man named Kaanthapuram and he is recogonised as the grand mufti of India (no one knows how he got that title).
As a whole, the organisation has a lot of flaws,but they are primarily responsible for upbringing the muslim community in Kerala, who were one of the most oppressed sections of society post and pre independence.They carry a baton of secularism and peace.IUML was somewhat responsible for championing equal treatment for muslims post independence and stopped the spread of radical islam .however, IUML leaders nowadays are mainly concerned about money and power,and their inaptitude has paved the way for chaos.
Mujahideen/Jamaat e Islami(Salafism,Wahabbism)-
these guys believe in salafism,a more puritanical form of islam.wahabbism is a sub-sect of salafis.their primary goal is to establish a nation run by islamic values,and aims to turn India into an islamic state.They consider secularism and democracry to be "haraam"(forbidden).many organisations that you mentioned-SDPI,PDP,WPI are all part of this sect. WPI is basically the political wing of Jamaat e Islami. SDPI is the political wing of NDF/popular front of india(classifeid as a terrorist organisation).SDPI can be described as the RSS of muslims.
They receive funding mainly from GCC countries,and they have used that money to buy a major influence in all forms of media(Media one news channel and Madhyamam newspaper are owned by Jamaat e Islami).they have declared solidarity with hamaas and some leaders even voiced their support for ISIS. they have less followers and acolyte compared to samasta. but if anything the last panchayath election showed us,is that their political power is rapidly expanding.