A planet is a large celestial body usually -- but not necessarily -- orbiting a star and not orbiting another planet, thus distinguishing planets from moons. As a general rule, planets are roughly spherical and have features like a core, crust and sometimes an atmosphere. However, the lack of a strong definition makes it hard to say what a planet is.
There are two primary classifications of planets. The first and most familiar to us is solid rocky bodies such as Earth and Mars. These planets are composed mostly of metals and siliconate materials. The other category of planets is gas giants such as Jupiter and Saturn. Such planets not totally made of gas, but most of their volume is made of gaseous elements like Hydrogen even if the tremendous pressure in their cores compress these gases into solid form.
There is no formal definition for what constitutes a planet at this time. The recent declassification of Pluto as a planet has shown the need for one and also how the lines can be fuzzy between a planet and smaller celestial bodies. Planets have been discovered orbiting stars other than our own. Some are far larger than Jupiter and thus seem to blur the line between planets and brown dwarf stars, thus further complicating the process of producing an official definition.