Question

I have googled this question, but I do not understand clearly what is an XML schema and DTD (document type definition), and why the XML schema is more powerful compared to DTD.

Any guidance would be highly appreciated.

Answer 1

From the Differences Between DTDs and Schema section of the Converting a DTD into a Schema article:

The critical difference between DTDs
and XML Schema is that XML Schema
utilize an XML-based syntax, whereas
DTDs have a unique syntax held over
from SGML DTDs. Although DTDs are
often criticized because of this need
to learn a new syntax, the syntax
itself is quite terse. The opposite is
true for XML Schema, which are
verbose, but also make use of tags and
XML so that authors of XML should find
the syntax of XML Schema less
intimidating.

The goal of DTDs was to retain a level
of compatibility with SGML for
applications that might want to
convert SGML DTDs into XML DTDs.
However, in keeping with one of the
goals of XML, “terseness in XML markup
is of minimal importance,” there is no
real concern with keeping the syntax
brief.

[…]

So what are some of the other differences which might be especially important when we are converting a DTD? Let’s take a look.

Typing

The most significant difference between DTDs and XML Schema is the capability to create and use datatypes in Schema in conjunction with element and attribute declarations. In fact, it’s such an important difference that one half of the XML Schema Recommendation is devoted to datatyping and XML Schema. We cover datatypes in detail in Part III of this book, “XML Schema Datatypes.”

[…]

Occurrence Constraints

Another area where DTDs and Schema differ significantly is with occurrence constraints. If you recall from our previous examples in Chapter 2, “Schema Structure” (or your own work with DTDs), there are three symbols that you can use to limit the number of occurrences of an element: *, + and ?.

[…]

Enumerations

So, let’s say we had a element, and we wanted to be able to define a size attribute for the shirt, which allowed users to choose a size: small, medium, or large. Our DTD would look like this:

<!ELEMENT item (shirt)>
<!ELEMENT shirt (#PCDATA)>
<!ATTLIST shirt
    size_value (small | medium | large)>

[…]

But what if we wanted size to be an element? We can’t do that with a DTD. DTDs do not provide for enumerations in an element’s text content. However, because of datatypes with Schema, when we declared the enumeration in the preceding example, we actually created a simpleType called size_values which we can now use with an element:

<xs:element name="size" type="size_value">

[…]