Imperative of sum - es or esto?

The imperatives of sum/esse (to be) are sg. es/esto and pl. este.

1. What is the difference between sum and est?
2. What conjugation is sum esse Fui?

What is the difference between sum and est?

Sum means "I am". Es means "You are". Est means "He is".

What conjugation is sum esse Fui?

sum, esse, fui, futurus- to be

Yes, sum does have third-conjugation-like features in the present tense. It is in the third conjugation that we can readily see versions of the thematic vowel e/o: DUC-e/o-o → duc-o-o → duco.