A Schengen Visa is the document issued for visiting/travelling in and within the Schengen Area. The Schengen Area is comprised of 26 countries that have agreed to allow free movement of their citizens within this area as a single country. If you intend to visit one of the Schengen countries for business, as a tourist or for a personal/private visit, you must first obtain a Schengen visa, unless your country is part of the Schengen territory or has a special agreement.
Visit the following link for further information : Schengen Visa Inforation
Your application should be lodged at the Embassy or Consulate at least 4 weeks before the proposed journey is undertaken.
For letter of invitation requirement need to write an email to organizer