A personal letter or a friendly letter is a type of letter which concerns  personal matters. It is sent from one individual to his/her friends, relatives,  parents, etc. Personal letters may help you to maintain and build personal  relationship with them. To understand what a personal letter is and how to  write the letter, you need to identify the social function, generic structure,  language features.
LEARNING ACTIVITY 2: INVITATIONS
INTRODUCTION
An invitation  letter is  a  document  that  presents  a formal  request for  the  presence  of  an  individual, a group of people or an organization at an event.  An invitation  may be formal or informal. It could be printed on paper or sent via  email. Invitations  can  be  used  for  a  variety  of  events  such  as  weddings,  graduation  ceremonies,  annual  dinners,  anniversary  parties  and  birthday  parties. An invitation usually contains some information about the host, the  date  and  time  of  the  event,  the  venue  of  the  event  and  how  to  accept  the  invitation. An invitation is usually sent many days in advance of the event,  to give the invitees an opportunity to respond to the invitation.
MAIN SECTION
2. Invitation  : a spoken or written request to somebody to do something or to go somewhere.
Generic Structure
Formal invitation letter
Sender's Name
 Sender's Address
 Date and letter's  sender