What is the difference between Ancestry and Descent?

The others here explained it. But basically ancestry is ascendancy (ascent). You're going backward in time, starting from you. You have two parents, four grandparents, eight great-grandparents, and so on.

Descendancy (descent) is the other direction. You start with a person or couple, like your great-grandparents, and then trace downward to living people. There's actually a term for this: "descendancy research." When you go backward in time, you know there have to be two parents for every ancestor, so you're always looking for a set number of people. In descendancy research, some descendants might not have children and may have never married, so you don't know exactly who you're looking for until you find them.

Ancestry

the people who were in your family in past times

Descent:

a : derivation from an ancestor

b : transmission or devolution of an estate by inheritance usually in the descending line

c : the fact or process of originating from an ancestral stock

Direction. Your ancestors are your parents, grandparents, great grandparents .... Your descendants are your children, grand children, great grandchildren...

You can cay "I am of Irish [Italian / Indian ...] descent" or "I have Irish [etc.] ancestors" and it means the same thing.

Ancestors are your parents, grand parents great grand parents and so on back........ descent ( descendants are your children, your grand child, your great grand children and so on forward