Partial Class

with the help of parial class, it is possible to split the definationof class in to two or more files. at the compiled time Compiler generate combine all partial class with same name.

Private, protected or protected internal cannot be explicity declared with in name space where class are define as partial.
yes you can any Access Modifiers with in partial class.

Example

file 1
    partial class AAA
    {
        private string ABC = "jignesh";
    }

file 2

 partial class AAA
    {
        public void b()
        {
            ABC = "tejas";
        }
    }

hope this will help you