When you write DataBinding syntax a method is created that gets called for
every row that will be created for your databinding control. Container is
a local variable in that method which is a reference to the row in the control
that was just created. DataItem is typically the property on the row object
that references the row from the original data source that the row is built
from.
A great way to understand what's going on is to do this:
<ItemTemplate><%# Container.DataItem.GetType().FullName %></ItemTemplate>
This will emit the type of object Container is. For different data bound
controls you'll see different output. Once you know the Container type, then
go look up in the docs about it.
