The Apostrophe (')ఏపోట్రాఫి

 It is used to indicate possessive [యొక్క] case i.e., to say something belongs to someone.
 [బహువ్రీహిలాగా]
 e.g. Rajasekhar's bag

In the case of plural nouns ending with 's' there is no need to add 's' after the
apostrophe. 
e.g. boys' bags, girls' books

In the case of other plural nouns NOT ending with "s'' both the apostrophe and "s'' are used. 
e.g. children's park, women's hostel, mice's squeaking

In the case of proper nouns [సర్వనామాలు]ending with  "s'' both the apostrophe and "s'' are used.
e.g. Thomas's dairy ,  Biswas's brother, Ashis's car,    Srinivas's wife 

The Apostrophe is also used to indicate missing letters in contracted [సంక్షిప్త]forms.
e.g.  He won't do it. (will not)
We shan't do such things (shall not)
Bread 'n butter is the sole reason for learning English. (and)
I'm hungry. (I am)
She's three. (She is/ has) ఆమెకు మూడేళ్లు or ఆమె దగ్గర మూడు ఉన్నవి 

Don't use the Apostrophe with 'it' in possessive [యొక్క  అనే  అర్ధం] cases.
The dog licked its paw.
The horse lifted its leg.
But ''it is'' అనే  అర్ధంలో  వాడినప్పుడు (’) వాడాలి 
  It's of no use going there. (It is)
  It's an interesting movie. (It is)
  It's a metal rod in it. (It has)
 
Also don't use the Apostrophe with yours.
Yours sincerely,  Yours faithfully, Yours truly….

Just check whether you have learnt the use!
the wheels of a car - car's wheels [JMýSP  [కారు  చక్రాలు] the wheels of several cars - cars' wheels [చాలా  కార్ల  చక్రాలు]
  the cloakroom of ladies - ladies' cloakroom
  the books of several boys - boys' books
  the tie of Charles - Charles's tie
  
Final check:
i)  Since the JRFs (Junior Research Fellows/Fellowships) progress was good he was awarded a SRF [Senior Research Fellowship].
ii) We have 6 JRFs in the department. 

Ans:
  i)  JRF's (We should keep (') after F) ఎందుకంటే  JRF  అనేది ఒక  వ్యక్తి.
  ii) JRFs (We should NOT keep (') after F, ఎందుకంటే  JRFs వ్యక్తులు