What is the difference in Angular between method= () and method()

Question

Sorry to ask this question, because it may disturb someone.

I wold like to know what is difference in Angular between

public someMethod= () => {
        logic...
}

and

public someMethod() {
        logic...
}

When to use first and when second and why? If I have a service for example, which one to use and why? Can I always use just someMethod()?

thank you