Stars and Planets are two different but very similar objects. Some of the properties that differentiate them are:

• Stars undergo nuclear reactions that burn hydrogen in their cores. For an object to have high enough temperature in its core to burn hydrogen, it needs to have a mass of at least 75 times of Jupiter. Anything smaller than that would be considered a planet.
• Stars form when a cloud of gas collapses under the influence of gravity. Planets form when material in the disk around a pre-existing star begins to condense around rock/ice cores.
• Planets orbit around stars or stellar remnants and have atmosphere while stars are massive luminous spheres of plasma that are held by their gravitational pull.
• Stars are usually made up of two gases, hydrogen and helium while planets are mainly made up of minerals and rocks.
• Planets orbit the sun so their location changes in the sky every night while stars appear to have a fixed location in the sky.

Early astronomers (without telescopes) differentiated between planets and stars mainly by looking at their position. Stars appear in fixed positions with respect to each other while planets move in complicated paths across the sky.