I'd just like to add to Jon's example. To get a reference to your own assembly, you can use:
Assembly myAssembly = Assembly.GetExecutingAssembly();
System.Reflection namespace.
If you want to examine an assembly that you have no reference to, you can use either of these:
Assembly assembly = Assembly.ReflectionOnlyLoad(fullAssemblyName);
Assembly assembly = Assembly.ReflectionOnlyLoadFrom(fileName);
If you intend to instantiate your type once you've found it:
Assembly assembly = Assembly.Load(fullAssemblyName);
Assembly assembly = Assembly.LoadFrom(fileName);
See the Assembly class documentation for more information.
Once you have the reference to the Assembly object, you can use assembly.GetTypes() like Jon already demonstrated.