describe breaks your test suite into components. Depending on your test strategy, you might have a describe for each function in your class, each module of your plugin, or each user-facing piece of functionality.
You can also nest describes to further subdivide the suite.
it is where you perform individual tests. You should be able to describe each test like a little sentence, such as “it calculates the area when the radius is set”. You shouldn’t be able to subdivide tests further– if you feel like you need to, use describe instead.
describe('Circle class', function() {
describe('area is calculated when', function() {
it('sets the radius', function() { ... });
it('sets the diameter', function() { ... });
it('sets the circumference', function() { ... });
});
});