For BDD Gherkin is a set of grammar rules used when you are writing a feature file while doing Behavior Driven Development. And it is meant to be wrote at a high level that non-programmers and non-testers could easily understand.

The grammar rules are used in a layout that uses keywords to be interpreted by Cucumber:

The most frequently used keywords are:

• Feature: A high level overview of what the following scenarios will do
• Scenario: Analogous to the title of a test case
• Given: The requirements or setup part of the test
• When: An action occurs
• Then: You are expecting the following to happen
• And: Can be used with Given, When, and Then to add additional requirements to each step

Other keywords are:

• Background: Give more context for the story
• But: Make sure something does not happen
• Scenario Outline: When you want to run the scenario multiple with different input values
• Ask: When you want to do semi-automation and involve the tester mid way through
• Given, When, Then, and And, all boil down to the same thing. The keywords are only in the document to make it more readable.

Example, when testing a simple calculator application

• Given: The application has started
• When: I press the 3 button
• And: I press the + button
• And: I press the 2 button
• And: I press the = button
• Then: I will be see 5 in the output

Summary

This was a quick article explaining that Gherkin is a simple set of grammar rules used with Cucumber.