Well Gunah(sin- Ma'siah) is, doing something against Allah orders non-intentionally(sometimes intentionally). And after that, becomes regretful and ask god to forgive. For example: Drinking alcohol,
But Kufr is different. Kufr means you do not believe in Allah and deny him.
Kufr is, a person deny Allah. However if that person becomes highly regret and return from the wrong way, and indemnify his bad works, and do Tawbah, God may accept his Tawbah. For example: A Muslim person changes his religion to Christian or Buddhism or get none religion.
But due to below Aya, God will forgive us for our Sin and Kufr, If we do a real Tawbah
قل: یا عبادى الّذین اشرفوا على انفسهم لا تقنطوا من رحمة الله أنّ الله یغفر الذنوب جمعیاً، إنّه هوالغفور الرّحیم؛
Say [that Allah declares,] ‘O My servants who have committed excesses against their own souls, do not despair of the mercy of Allah. Indeed Allah will forgive all sins. Indeed, He is the All-forgiving, the All-merciful. 39:53
