Mass is one of the most important characteristics of a star because a high mass is what causes the conditions in the core to be appropriate for nuclear fusion. Stars vary considerably in their mass: the sun is considered to be an average size star (the sun is the scale by which other stars are measured; it weighs one solar mass), but some stars are much larger (upwards as 10 times more massive), while some are not much larger than the planet Jupiter.
Temperature is directly related to mass in a star: the more massive it is, the denser the core, and thus the higher the temperature. The temperature in the core is so high that it causes hydrogen atoms to separate into protons and electrons, where they swirl around in a plasma medium. The temperature in the sun is around 5,000 degrees Kelvin (about 8,500 Fahrenheit), while some are as hot as 25,000 degrees (around 45,000 degrees Fahrenheit).
Brightness is a measure of how much power the star releases; it can also be used as a measure of how visible a star is from earth. The former is a direct factor of temperature and mass (the more massive a star is, the more power it releases), while the latter is more a factor of distance. A star's brightness is measured by magnitude; stars are placed on the H-R Diagram based on their magnitude. Nearly 90 percent of stars are classified as Main Sequence stars on the diagram, which are stars that are about as bright as the sun.
Stars are born when gravity causes large clumps of gas and interstellar matter congeal together; this is when a star's mass is determined (and thus temperature and brightness): the larger the initial clumps, the larger the star will be. A star's life lasts as long as there is hydrogen in the core to be fused into helium; once a star runs out of hydrogen, it begins to die. How a star dies depends on its mass: smaller stars become hot, dense shells called white dwarfs, while larger stars explode into supernovas.