Good question! A nebula is a spinning disk- shaped body of cosmic “dust” and gas, and it contains the building blocks of a solar system. Our solar system (the sun and the planets and smaller bodies) formed from a nebula. I guess that a nebula can be considered the first stage of a star’s life cycle, though the star is not around yet when the nebula first forms. Hydrogen and Helium are the most abundant elements in a solar nebula. Gravitational attraction causes a massive amount of these gases to accumulate in the center of the nebula. When the ball of gas (soon to be a star) gets bigger and denser, the gravitational energy is turned into heat. When the ball of gas gets hot enough (about 10 million degrees Celsius), hydrogen starts to convert to helium in a nuclear reaction. This is a process called hydrogen burning, and this is the stage that the sun is in now. The sun probably reached this stage about 1 million years after the formation of our solar nebula (Dickin, 2005, and references therein) and has been going steady for about 4.55 billion years!

reference
Dickin, A.P. (2005). Radiogenic Isotope Geology: 2nd edition. New York: Cambridge University Press.

A nebula is n cloud of gas, plasma, dust, or some combination thereof, in space. Nebulae with enough mass can collapse under their own gravity to become stars - this is how all stars are born. That said, some nebulae have much less mass, such as those created by stars as they die, and they cannot become new stars (by themselves).

A nebula is a cloud of gas and dust, from which by gravitational collapse a star is born.

A nebula is simply a large body of gas and dust in the solar system. Astronomers have identified many different types of nebulae. Indeed, one type of nebula, a solar nebula, describes the collapsing cloud of gas and dust that leads to the birth of a star. However, another type of nebula can be created during other points in a star's life cycle. For example, a planetary nebula is the cloud of dust and gas ejected from a dying star during its supernova phase.

fun fact: nebula comes from the latin word for 'cloud'.