The definition of DTD is relatively simple. In DTD, we can define the nested hierarchy between elements via Element, which is the Structure of XML, and we can define the content of a single element via attlist, which is a DataType of an element. The shortcoming of DTD is that he is not flexible and scaled, and the description is not strong. Schema is also strengthened in these aspects. Of course, SCHEMA is not defined to replace DTD, and Schema's goal is much larger than this. The definition of Schema is more complicated than DTD. First, you must figure out the difference between Simple Content, Complex Content, Simple Type, and Complex Type. It is then distinguished that simple type is used to define the definition of the data type, but Simple Type is more powerful than the definition in the DTD. The first is the foundation type system, this is DTD, then still There is a custom extension mechanism, but also the regular expression, so DTD is completely unable to follow this ratio. Complex Type is used to define structures, like Element, but it is obviously better than this, and at least use Minoccur and Maxoccur, and you can use Sequence, Choice, All to explain the organizational order. There is also a feature that provides importance.
