Although unit & integration tests serve different purposes, we’ve a tendency to confuse both types of testing. In fact most of the tests we write tend to be integration tests. In my opinion the main reason why we confuse both is because we use the same test automation framework (e.g. NUnit) to write unit tests and integration tests. Nevertheless we should always separate our unit tests from our integration tests because Integration tests tend to be more fragile, slower and require more maintenance as unit tests.
As described in Wikipedia integration tests, test the integration between modules. Unit Tests targets atomic (indivisible) units/modules. In my opion the notion of module is not enough to separate Unit from integration tests because a module is a subjective concept, it can be a applied for many things; a class, a Layer, a Component… Therefore I prefer to make the distinction based on the fact that the test is dependent or not on some infrastructure. When our tests are dependent on some infrastructure we can’t pretend anymore that our test is exercising a single module. So as soon as our test is dependent on some sort of infrastructure like a DB, file, Web Service, COM component… it’s depend on at least two units (our code & the infrastructure) and it should be qualified as an Integration test.