Magnetized plasmas accelerated and flowing out from a gravitational potential
are well observed on very different astrophysical scales. Such outflows also
show very different morphologies going from the almost spherically
expanding solar wind to the very tightly collimated jets from young stars and
extra-galactic sources. Thus the presence of jets is probably linked with the
existence of accretion discs but accretion alone does not explain the
physical processes that take place in the outflowing wind to eventually form
a jet.
We shall briefly review last developments in semi-analytical modelling of the
formation of steady winds and how they may help understanding time dependent
numerical simulations. In particular we
shall discuss the necessity for such flows to be efficiently accelerated
in order to be causaly disconnected from their source. We shall also compare
self-similar stellar outflows with self-similar disc wind models.
Special emphasis will be put on constructing criteria to predict how different
boundary conditions lead to the formation of jets or winds. Such criteria
show that jets may be either pressure or magnetically confined. One of the most
exciting and promising possibility is that the confinement is equally due
to the pressure and the magnetic field though these two sources of
confinement are usually considered separately. In conclusion we may conjecture
on the evolution of jets into winds as stars reach the main sequence and ask
what is the structure of the present solar wind.