Definition: FAST Query Language (KQL) is used to form search criteria in SharePoint

FQL is structured query language which supports advanced query operators. FQL is used when you want to create complex queries programmatically.

Some Points:

• FQL is not intended to be exposed to end users and it is disabled by default. To Enable FQL, use EnableFQL
• FQL is used to form queries in SharePoint and most of the time they are used to form queries programmatically.
• FQL has some more capabilities than KQL
• FQL is a bit more difficult to execute using UI or any API. So most of the time developers prefer to construct and execute queries using KQL
• The limit of FQL query is 2048 characters.