Iman is the main pillar of Islam. The meaning of Iman is faith.
To have faith heart and soul in basic aspects of Islam such as belief in One Allah, His prophets & messengers and Asmani Kitab is called Iman.
The person who has Iman is called Mumin or believer.
Kalima Tayyiba is the basis of Tawhid, Iman and Islam.
The meaning of Kalima Tayyiba is ‘the holy sentence’. Nobody can make his or her entry into Islam without accepting this Kalima.

Kalima Tayyiba

لَا اِلَهَ اِلاَّ اللهُ مُحَمَّدُ رَّسُوْ لُ الله

Pronunciation: La Ilaha Illallahu Muhammadur Rasullullah.

Meaning: There is no Mabud except Allah. Hazrat Muhammad (SM) Allah’s Rasul.

On the other hand, Kalima Shahadat is the ‘word of testimony’. That means, testimony to Iman is given with this Kalima. By believing in Allah and reciting 'Kalima Shahadat’, one has to achieve Iman on Allah.

Kalima Shahadat

اَشْهَدُ اَنْ لَّا اِلَهَ اِلَّا اللهُ وَحْدَهُ لَاشَرِيْكَ لَهُ وَاَشْهَدُ اَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُوْلُه

Pronunciation: Ashhadu Al La-Illaha Illallahu Wahdahu La-Sharika Lahu, Wa Ashhadu Anna Muhammadan Abduhu Wa Rasuluhu.

Meaning: I testify that there is indeed no Deity then Allah. He is one and He has no partner. I also testify that Muhammad (SM) is Allah’s servant and messenger.

Therefore, we shall utter the Kalima correctly and act according to its inner meaning.