Types of Data

3 types of data exist:

Structured data:

Until recently written by hand and kept in notebooks, this data is now stored electronically on spreadsheets and databases, and consists of spreadsheet-style tables with rows and columns, each row being a record and each column a well-defined field (name, date of birth…).
We are contributing to these structured data when, for example, we provide the information necessary to order goods online.
Carefully structured and tabulated data is relatively easy to manage and is amenable to statistical analysis, indeed until recently statistical analysis methods could apply only to structured data.

Unstructured data:

By contrast, unstructured data is not so easily categorized. It includes:
videos
photos
tweets
word-processing documents
Once the use of the world-wide web became widespread, it transpired that many such potential sources of information remained inaccessible because hey lacked the structure needed for existing analytic techniques to be applied.

Semi-structured data:

However by identifying key features data that appears initially to be unstructured may not be completely without structure.
Emails for example, contain for example structured metadata in the heading as well as the actual unstructured messages in the text so it may be classified as semi-structured data.
Metadata tags, which are essentially descriptive references, can be used to add some structure to unstructured data.
Adding a word to an image on a website makes it identifiable and subsequently easier to search for. Semi-structured data is also found on social networking websites, which use hashtags so that messages (which are unstructured data) on a particular topic can be identified.
Dealing with unstructured data is challenging: since it cannot be stored in traditional databases or spreadsheets, special tools have to be developed to extract useful information.

Reference: Big Data: A very short introduction by Dawn E. Holmes. Oxford University Press, 2017