When to use properties instead of functions

This is probably a matter of personal preference, but when do you use properties instead of functions in your code

For instance to get an error log I could say

string GetErrorLog()
{
      return m_ErrorLog;
}

or I could

string ErrorLog
{
     get { return m_ErrorLog; }
}

How do you decide which one to use? I seem to be inconsistent in my usage and I'm looking for a good general rule of thumb. Thanks.