Data is largely classified as Structured, Semi-Structured and Un-Structured.
If we know the fields as well as their datatype, then we call it structured. The data in relational databases such as MySQL, Oracle or Microsoft SQL is an example of structured data.
The data in which we know the fields or columns but we do not know the datatypes, we call it semi-structured data. For example, data in CSV which is comma separated values is known as semi-structured data.
If our data doesn't contain columns or fields, we call it unstructured data. The data in the form of plain text files or logs generated on a server are examples of unstructured data.
The process of translating unstructured data into structured is known as ETL - Extract, Transform and Load.