<p>In C#, what makes a field different from a property, and when should a field be used instead of a property?</p>

Properties expose fields.  Fields should (almost always) be kept private to a class and accessed via get and set properties.  Properties provide a level of abstraction allowing you to change the fields while not affecting the external way they are accessed by the things that use your class.
```
```
public class MyClass
{
    // this is a field.  It is private to your class and stores the actual data.
    private string _myField;

    // this is a property. When accessed it uses the underlying field,
    // but only exposes the contract, which will not be affected by the underlying field
    public string MyProperty
    {
        get
        {
            return _myField;
        }
        set
        {
            _myField = value;
        }
    }

    // This is an AutoProperty (C# 3.0 and higher) - which is a shorthand syntax
    // used to generate a private field for you
    public int AnotherProperty { get; set; } 
}
```
```
@Kent points out that Properties are not required to encapsulate fields, they could do a calculation on other fields, or serve other purposes.
@GSS points out that you can also do other logic, such as validation, when a property is accessed, another useful feature.