What is the difference between DescendantAdded and ChildAdded?

One Deals With Direct Children and One Deals With Children In Any Form Of Hierarchy.

ChildAdded only deals with a hierarchy directly under the object while DescendantAdded deals with a form of child added to the hierarchy.

For example, let's take a look at an example:

workspace.ChildAdded:connect(function(obj)
	print(obj.Name)
end)

workspace.DescendantAdded:connect(function(obj)
	print(obj.Name)
end)

s = Instance.new("Part")
s.Name = "Steve"
s.Parent = workspace
bob = s:Clone()
bob.Name = "Bob"
bob.Parent = s

The model structure of this looks like the following:

-Workspace

Bob [Under Workspace]

Steve [ Under Bob]

The following code will print:

Steve

Steve

Bob

The Steve's name is printed twice because it is a direct child of Workspace, thus DescendantAdded and ChildAdded both detect it.

However, Bob's name is only printed once from DescendantAdded and not from ChildAdded because it's not a direct child but is yet in the hierarchy.

REAL LIFE ANALOGY:

Let's say Family Tracking Company 1 is ChildAdded and Family Tracking Company 2 is DescendantAdded;

If John And Mary Had A Child Named "Steve"[I won't tell you the process of how a child is made]:

Both Company 1 and Company 2 would recognize "Steve" and track him. However, say "Steve" and a girl named "@sera plz" had a baby and the babies name was "DigitalVeer":

Company 1 wouldn't track that baby as it is a 'grandchild' and not a direct child but Company 2 would.