For a classification of stars see The H-R Diagram.
The most accepted theory is that stars are formed from molecular clouds of gas and dust, that collide under their own gravity. Theory also suggests that in rare cases stars can be formed from Brown Dwarfs or even giant planets if somehow they accrete matter until they have enough mass to fuse hydrogen.
Formation[]
When a cloud of gas and dust collides under its own gravity, it condenses, forming a protostar. Then when the cloud is cleared out and the star becomes visible, we have a pre-main sequence star. However, this works only for stars like our Sun or smaller. Larger objects, like the O - type stars, might reach maturity (main sequence) before they clear out the molecular cloud.
Protostars[]
Since protostars are surrounded by the cloud they are feeding on, it is impossible to see them clearly, so we don't know exactly how they look. Theoretical models suggest they are not hot and still don't produce visible light, or if they do, that light is dark red. It looks like these stars are a bit (but not much) larger then the stars they will become.
In the core of a protostar, hydrogen fusion is not occurring yet. Deuterium might be fusing, but released energy is only used to prevent the core from collapsing, giving the star enough time to accrete more matter. Once hydrogen fusion starts, the star brightens and pushes away the remaining cloud surrounding it.
Pre main sequence star[]
For stars like our Sun or smaller, once they have finished consuming the molecular cloud surrounding them or once they have ejected the cloud (and hence became visible), they are considered pre-main sequence stars. For larger stars, who evolve much faster, they reach maturity before they clear the surrounding cloud. Pre-main sequence stars are larger and usually brighter then what they will become. Hydrogen fusion might or might not have started in the core. Their energy comes from gravity, from compressing the layers of gas that composes them. If hydrogen fusion has started, the energy output is still lower then the energy they radiate.
Surrounding environment[]
Protostars are surrounded by the molecular cloud they are feeding on. Planets have not formed yet. Still, it is possible for a passing planet to be captured by their gravity. In that case, the planet will also accrete matter from the cloud.
Planets[]
In case of pre-main sequence stars, it is questionable if they have planets. M - type stars, which have a very long life (and also a long pre-main sequence lifetime), could have enough time for planets to develop around them during this phase.
If planets are to be found around such stars, they are hot and volcanic.
Still, there is one possibility of a passing planet, captured by the gravity of a young star. Such a planet might be suitable for terraforming. However, around a forming solar system, there should be a high risk of collisions. The cloud of gas and dust surrounding the star will also influence the planet and all spaceships flying towards it.
Colonization[]
What reason would someone have to settle around a young star?
Industry. In case of mature solar systems, nearly all heavy elements are condensed in the cores of planets, so mining them will be more expensive. Also, many volatiles, like helium, are only found in the star and in the atmospheres of giant planets, where mining could be very difficult. On the other hand, the environment surrounding young stars is very dangerous and challenging.
Piracy. It could be plausible that future space pirates will prefer to hide in the clouds surrounding young stars. There, they have a lot of radio interferences that can camouflage their ships.
Tourism and research. Scientists will be very interested to see with their own eyes how stars and planets are born. Also, the incredible views will attract many tourists.
Young stars are not expected to have planets suitable for terraforming. Still, they offer a unique and challenging environment.