What is the difference between Msol and Azure AD?

As a financial and cryptocurrency practitioner, I often encounter various cloud-based solutions in my work. Among them, Microsoft's Msol and Azure AD are two commonly mentioned terms. However, I'm curious to understand the key differences between the two. Could you elaborate on what Msol stands for and its specific role? Conversely, what is Azure AD, and how does it differ from Msol? Understanding these nuances is crucial for me to make informed decisions when selecting cloud services for my financial and cryptocurrency-related projects. Your insights would be invaluable in clarifying these concepts.