They need to be Serializable and public. See the following snippet:

using UnityEngine;
using System;

[Serializable]
public class Pair
{
    public int x;
    public int y;
}

Only classes that inherit from MonoBehaviour can be attached to GaneObjects. That's a limitation of Unity, not of a specific language. In UnityScript you can omit the class construct and Unity will inherit your class automatically form MonoBehaviour. In C# you have to do that yourself. If you talk about custom data classes and you have a vatiable on a MonoBehaviour that is holding a class instance, to see it in the inspector your have to add the attribute System.Serializable.

In Unityscript all classes are implicitly serializable.