Sorry you sreejith for keeping you waited a long time.
Please refer to pages 105 for potentiometer, 109 for rheostat, book "Teach yourself electricity and electronics", McGraw Hill. Many books define the same, but I just give you the name of the book from McGraw Hill, one of the most famous publisher, especially in practical action.
Between potentiometer and rheostat, there is a mixed using in daily using or in the different literatures, so just make clear the definitions, and understand what people are talking about.
Goodluck
nguyennam