I will talk about Semantic Web because I think it offers the most advanced studies and language implementations about the subject.
Resource Description Framework is one of the many data models inherent to Semantic Web available to describe informations.
RDF is an abstract model with several serialization formats (i.e.,
file formats), and so the particular way in which a resource or triple
is encoded varies from format to format
and
However, in practice, RDF data is often persisted in relational
database or native representations also called Triplestores, or Quad
stores if context (i.e. the named graph) is also persisted for each
RDF triple.
RDF content can be retrieved using RDF Queries.
Topic Maps another model of knowledge data storing and representation.
Topic Maps is a standard for the representation and interchange of
knowledge, with an emphasis on the findability of information.
and
In the year 2000 Topic Maps was defined in an XML syntax XTM. This is
now commonly known as "XTM 1.0" and is still in fairly common use.
From the official Topic Maps Data Model:
The only atomic fundamental types defined in this part of ISO/IEC13250
(in 4.3) are strings and null. Through the concept of datatypes, data
of any type can be represented in this model. All datatypes used shall
have a string representation of their value space and this string
representation is what is stored in the topic map. The information
about which datatype the value belongs to is stored separately, in the
form of a locator identifying the datatype.
There are many other formats proposed, you can take a look at this article for more informations.
I also want to link you a recent answer I wrote about a similar topic with a lot of useful links.
After reading various articles, I think a common direction every method is taking is storing data as a text format. The relative information can be stored in a database directly as text.
Having the data in an understandable text format has several benefits, perhaps more than the disadvantages.
Other Semantic methods such as Notation 3 (N3) or Turtle Syntax use slight different formats, but still plain text.
A N3 example
@prefix dc: <http://purl.org/dc/elements/1.1/>.
<http://en.wikipedia.org/wiki/Tony_Benn>
dc:title "Tony Benn";
dc:publisher "Wikipedia".
Finally, I would like to link you an useful article you should read: Standardization of Unstructured Textual Data into Semantic Web Format.