The earliest stars probably formed only a few hundred thousand years after the Big Bang that started the universe - maybe about 13.5 billion years ago. Stars are the oldest big objects in the Universe.
Stars formed big clumps of hydrogen and a few helium atoms, the simplest kinds of atoms, and pretty much the only kinds that existed before there were stars. Once enough hydrogen and helium atoms clumped together, though, they began to have tremendous gravity.
This gravity pulled the hydrogen atoms closer and closer together in the middle of the star. All those electrons in one place made the star hotter and hotter - about 100 million degrees celsius. As the atoms got closer together, they bumped into each other more often. When this happened, sometimes the two atoms would join together into one atom, and two hydrogen atoms would turn into one helium atom. We call this process "nuclear fusion". Nuclear fusion releases a lot of extra electrons, and these electrons go shooting off into space in all directions. It's those zillions of electrons that you see, that make the stars shine - and that's what makes our Sun shine, too. The electrons give off all wavelengths of energy, so they give off not only light, but also heat.
When a star runs out of hydrogen atoms, the helium atoms begin to join up and make carbon atoms. When the star runs out of helium atoms, the carbon atoms begin to join up and make oxygen, and so on until the heaviest element stars can make, which is iron. It takes a lot of energy to make these heavier atoms, so it can only happen inside stars, which make a lot of energy. The gravity of stars also pulled them together into groups, so that they formed into galaxies.
Today there are about 1022 to 1024 stars in the universe (click here to find out what 1022 means). New stars are still being born every day (and old ones die). But even that many stars don't do much to fill up the Universe - most of the Universe is still just empty space.
Most stars spin around on their axis, just like the Earth does - like a top. This is because the nebula the star was made of also rotated. And most stars also orbit through space in a large circle around the center of their galaxy, taking millions of years to go around even once.
In the daytime, you can't see the stars from the Earth. They're still there, but when the Sun is on your side of the Earth, it sends us so much light that it drowns out the little lights of the stars.
Bibliography and further reading about stars