when did the need for a definition arise?
In January 2005, with the discovery of the trans-Neptunian object Eris.
Definition of planet
The issue of a clear definition for "planet" came to a head in January 2005 with the discovery of the trans-Neptunian object Eris, a body more massive than the smallest then-accepted planet, Pluto.
give any three differences between planets and stars Planet -  it is round or has planetary geology; "moves" like a planet. Star - an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity.
wiki:304985
Definition of planet [SEP] Introduction
These may be characterized as the orbital definition and the geophysical definition. The issue of a clear definition for "planet" came to a head in January 2005 with the discovery of the trans-Neptunian object Eris, a body more massive than the smallest then-accepted planet, Pluto. In its August 2006 response, the International Astronomical Union (IAU), recognised by astronomers as the world body responsible for resolving issues of nomenclature, released its decision on the matter during a meeting in Prague. This definition, which applies only to the Solar System (though exoplanets had been addressed in 2003), states that a planet is a body that orbits the Sun, is massive enough for its own gravity to make it round, and has "cleared its neighbourhood" of smaller objects approaching its orbit.