The Difference Between Islam and Iman
In Islamic theology, there is a distinct difference between the concepts of Islam and Iman. This distinction is beautifully outlined in the famous "Hadith of Jibril" (Gabriel), where the Angel came to the Prophet Muhammad (ﷺ) in the form of a man to teach the companions their religion.
The Pillars of Islam (Actions)
When asked about Islam, the Prophet (ﷺ) responded with the 5 physical pillars: Testifying that there is no God but Allah (Shahadah), performing the daily prayers (Salah), paying charity (Zakat), fasting in Ramadan (Sawm), and performing Hajj. Islam deals with our outward actions and submissions.
The Articles of Faith (Beliefs)
When Jibril then asked, "Tell me about Iman," the Prophet (ﷺ) responded with the 6 Articles of Faith listed above. Iman deals with the inward state of the heart and mind. You cannot have true Islam without Iman, and your Iman must be manifested through the physical actions of Islam.
A person's Iman is not a static state; according to Sunni theology, Iman increases with obedience to Allah and decreases with disobedience.