Sunnah Prayer

What is the name of the sunnah prayer that we have to pray when we enter the mosque ,and is it considered as sunaah only when people are gathering for a congregational prayer i.e. when we am just in town shopping and stop for any fard prayer at the mosque can i enter and pray the 2 rakha first?

As-Salamu `alaykum. Praying 5 times a day is a must for every Muslim. But when it comes to praying Sunnah after Prayer sometimes I struggle. Can you please relate any hadith, and/or benefits gained when performing Sunnah Prayers after obligatory Prayers?

Dear Scholar,Assalam alykum Wr Wb. First of all, I'd like to know the difference between the Fardh, Sunnah (al moakida and others)and Witr rakats in salah. Secondly, I'd like to know if its a sin to not pray sunnah and witr rakat. I have heard that any person who prays just fardh rakat and misses sunnat e moakida or witr rakat is a sinner. Is it true? I think that only not observing fardh is a sin, then how can we say that not performing sunnah is also a sin? Please clarify and explain in detail. Thank you for your valuable help. Jazakallah