A star undergoes nuclear reactions that burn hydrogen and it forms when a cloud of gas collapses under gravity. Planets do not experience nuclear reactions, nor do they form from a cloud of gas.
Anything in the universe with a mass of more than 75 times the size of Jupiter automatically gets star status. A star must also have a high enough temperature to allow nuclear fusion to take place.
Planets form when materials around a star begin to condense. These materials often condense around rock or ice cores and some planets are almost completely rock and ice while others are gas surrounded by rock and ice.