There are literally trillions upon trillions of stars in the observable universe. Over the thousands of years humans have been observing, noting down and categorising stars we have begun to understand that they have a few main classifications. Excluding stellar remnants (such as neutron stars and black holes) and quasi sub stellar objects (such as proto stars and brown dwarfs), most stars fit into a few categories. Generally, the reddest stars are relatively cooler and the bluer the star the hotter it is. There are huge number of categories for stars, influenced by luminosity (brightness), mass and radial size.
The following two are the most basic of them but should not be taken as categorical.
This category covers the majority of observed stars in our galaxy (including our Sun), and thus the cosmological principle holds that this should be true for other galaxies as well. Main Sequence stars are big enough that their internal heat and pressure causes hydrogen to fuse in their core, creating the stellar fusion that gives them their phenomenal power. The lower size limit for a main sequence star is about 0.08 times that of the Sun, or 80 times that of Jupiter. Below that, they are not big enough to ignite nuclear fusion and are classified as a Brown Dwarf.
Main sequence stars can grow as big as 100 suns – any larger than this and they are classified as a giant stars. Many Main Sequence stars will burn through their core hydrogen supply eventually, causing them to start fusing helium. This rapid temperature increase causes the star’s gaseous outer layer to ignite and expand, creating a giant star. Smaller or cooler stars may undergo convection, shedding their outer layers rather than expanding – these directly become white dwarfs. However, no main sequence star we can observe has been around long enough for us to have seen this occur.
Red or blue giants are older stars that have burned through much of their hydrogen cores and swell up as the outer layers of hydrogen become their new fuel. Inside, they begin to fuse helium and other heavier elements. In order to fuse helium, a star must be about 0.8 solar masses or above. Thus, becoming a red giant is the eventual fate of our Star.
Some of the biggest giant stars can dwarf our Sun by literally thousands of times. UY Scuti, the most likely candidate for the largest star by radius, is about 1708 times larger than our Sun. The biggest stars will continue to fuse elements, going up the periodic table, until they reach iron – which is an extremely stable element that is difficult to fuse. An iron core will build up at the heart of the star over billions of years, which eventually becomes so heavy that the nuclear reactions cannot counteract the force of gravity.
Thus, the most likely fate for stars of this size is to end in a colossal explosion known as a supernova. Luckily, astrophysicists have calculated that our Sun is likely to escape this catastrophic end to its several-billion-year life span. However, Earth will be long gone by that time as it’s orbit will be swallowed up by the expanding red giant that once gave it life.