What is the difference if TestFixture attribute is used or not

Is purely a convenience, beginning with NUnit 2.5, if a class satisfies the conditions to be a test fixture and specifies at least a method marked with Test, TestCase or TestCaseSource than that class is treated as a test fixture.

The TestFixture attribute is required however for parameterized or generic test fixture because in that case you also specify additional information through the attribute (parameters/concrete types).

More information about the specific requirements is available at TextFixtureAttribute (NUnit 3 Documentation) and TestFixtureAttribute (NUnit 2.0 / 2.5)

If you use [TestFixture] you have to put up with that voice in the back of your head saying "Hmm, this is a test class, not really a test fixture" and wonder how they got this annotation wrong.

See Why testFixture instead of testClass.