Answer:
Stars are gigantic spheres of nucleosynthesis. They start off as hydrogen mainly but gradually fill up with heavier elements as they synthesise them.
Stars form from nebulae, vast clouds of hydrogen. Pockets of gas in the nebulae ball into spheres by gravity. Through tunnelling (relies on the Uncertainty Priciple of quantum theory), hydrogen nuclei fuse to form helium nuclei and this reaction gives of heat. Millions upon millions of these reactions heat up the star. Hydrogen is converted in this way (nucleosynthetically) into helium and helium into carbon.
A star is a 'delicate balance'. The heat from the core pushes outwards and the mass of the star tries to push the outer layers of the star inwards in collapse. Paradoxically, the larger a star the shorter is the time until it exhausts its nuclear 'fuel'. The nucleosynthetic reactions stop at iron. Energy must be added to produce elements heavier than iron.
The largest stars (much larger than the Sun) explode as supernovae. The supernova energy is enough for the synthesis of the heavier elements, from iron to uranium.
With no nucleosynthetic energy continuing to be generated, the core collapses. Stars can shrink into white dwarves (where collapse is halted by electron degeneracy) or neutron stars (where collapse is halted by neutron degeneracy), or black holes, where collapse cannot be halted if the mass of the post-supernova core is above a certain limit - the Chandrasekhar limit.
There are a couple of videos on youtube about size and scale in the Universe. They start with the Moon and then show larger and larger and larger objects, from the Sun to Sirius and Aldebaran and the largest star so far known VY Canis Majoris.
The Sun is a star 150 million km from Earth and has a diameter of 1 390 000 km. The nearest star to the Sun is Proxima Centauri and is 4.3 light years from Earth.
Stars can be different colours, depending on their temperatures. Cool stars (from 3500 degrees Celsius) are redder in colour. The hottest stars are blue stars (up to 50 000 degrees Celsius). The Sun is 6000 degrees Celsius on the surface and 15 000 000 degrees Celsius at the centre and is classified as a type G yellow dwarf star.
Stars are classified by their size and colour on the Herzsprung-Russell Diagram.
Stars (in their billions) form huge groups (galaxies) thousands of light years in diameter. Galaxies have several shapes (spiral, barred spiral, elliptical and irregular).