I haven't seen much information online comparing the similarities and differences between the Nunit and MSTest Frameworks.  Here I will define the similarities and some of the differences.  If there is anything else which you come upon, please do add it to the comments.

MSTest Attribute

NUnit Attribute

Purpose

[TestMethod]

[Test]

Indentifies of an individual unit test

[TestClass]

[TestFixture]

Identifies of a group of unit tests, all Tests, and Initializations/Clean Ups must appear after this declaration

[ClassInitialize]

[TestFixtureSetUp]

Identifies a method which should be called a single time prior to executing any test in the Test Class/Test Fixture

[ClassCleanup]

[TestFixtureTearDown]

Identifies a method in to be called a single time following the execution of the last test in a TestClass/TestFixture

[TestInitialize]

[SetUp]

Identifies a method to be executed each time before a TestMethod/Test is executed

[TestCleanUp]

[TearDown]

Identifies a method to be executed each time after a TestMethod/Test has executed

[AssemblyInitialize]

 N/A

Identifies a method to be called a single time upon before running any tests in a Test Assembly

[AssemblyCleanUp]

 N/A

Identifies a method to be called a single time upon after running all tests in a Test Assembly

 The order of execution is similar in both frameworks, but there are some differences between the two:

  • In Nunit, tests are not executed in parallel.  Rather, it appears that all tests execute on a single thread.  In MSTest, each test is instantiated on a separate thread, this results the runs being interleaved.  Therefore, if test A depends on test B for its success, it likely will fail as test B will likely start running as test A is running.
  • The time at which ClassCleanUp/TestFixtureTearDown executes is different between the two frameworks.  In Nunit, TestFixtureTearDown is executed immediately following the completion of the last test in a TestFixture or after TearDown if the attribute exists for the test in question.  In the Whidbey implementation of MsTest, ClassCleanUp executes at the end of a test run, before AssemblyCleanUp but not necessarily immediately after the last test in a TestClass has completed executing.
  • In Whidbey, support for test class inheritance was missing.  In Nunit, it is fully supported.  This will be rectified in Orcas.

I should also mentioned that in MsTest, TestContext exists for passing information about the test run.  There is no equivalent in Nunit tests.  This can serve as a handy tool for pulling information from datasources on the disk to the unit tests, as well as other uses.  More can be read about it here.