What are the Illiyin and the Sijjin?

Assalaamu Alaikum wa Rahmatullahi wa Barakatuhu,

All the deeds that we do are recorded in 'registers' by angels appointed to be with us all the time. They record all our deeds, good and bad.
The illiyun is the record of deeds of the good people that will be presented to Allah Ta'ala on the day of judgement and the Sijjin is the record of deeds of the wicked. Ref S83 v18-20 and S83 v 7-9 respectively.

Then he who is given his record in his right hand
Soon will his account be taken by easy reckoning
And he will turn to his people rejoicing!
But he who is given his record behind his back
Soon will he enter perdition
And he will enter a Blazing Fire.
S 84 v 7-12